202109101417 Exercises 1.1.A (Q5)

By equation (1.1), \pi =4(1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\cdots ), where the n^{\textrm{th}} term in the sum inside the parenthesis is \frac{(-1)^{n+1}}{2n-1} (starting at n=1). So the first approximation of \pi using this formula is \pi\approx 4(1)=4.0, and the second approximation is \pi\approx 4(1-\frac{1}{3})=8/3\approx 2.66667. Continue like this until two consecutive approximations have 3 as the first digit before the decimal point. How many terms in the sum did this require? Be careful with rounding off in the approximations.


Attempts.

1^{\textrm{st}} approximation:

\pi\approx 4(1)=4.0

2^{\textrm{nd}} approximation:

\pi\approx 4(1-\frac{1}{3})=\frac{8}{3}\approx 2.66667

3^{\textrm{rd}} approximation:

\pi\approx 4(1-\frac{1}{3}+\frac{1}{5})=\frac{52}{15}\approx 3.466667\quad (\textrm{5 d.p.})

4^{\textrm{th}} approximation:

\pi\approx 4(1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7})=\frac{304}{105}\approx 2.89523\quad (\textrm{5 d.p.})

5^{\textrm{th}} approximation:

\pi\approx 4(1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9})=\frac{1052}{315}\approx 3.33968\quad (\textrm{5 d.p.})

6^{\textrm{th}} approximation:

\pi\approx 4(1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11})=\frac{10312}{3465}\approx 2.97605\quad (\textrm{5 d.p.})

7^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}\bigg) \\ & =\frac{147916}{45045} \\ & \approx 3.28374\quad (\textrm{5 d.p.}) \end{aligned}

8^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}\bigg) \\ & =\frac{135904}{45045} \\ & \approx 3.01707\quad (\textrm{5 d.p.}) \end{aligned}

9^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}\bigg) \\ & =\frac{2490548}{765765} \\ & \approx 3.25237\quad (\textrm{5 d.p.}) \end{aligned}

10^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}\bigg) \\ & =\frac{44257352}{14549535} \\ & \approx 3.04184\quad (\textrm{5 d.p.}) \end{aligned}

11^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}\bigg) \\ & =\frac{47028692}{14549535} \\ & \approx 3.23232\quad (\textrm{5 d.p.}) \end{aligned}

12^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23}\bigg) \\ & =\frac{1023461776}{334639305} \\ & \approx 3.05840\quad (\textrm{5 d.p.}) \end{aligned}

13^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}\bigg) \\ & =\frac{5385020324}{1673196525} \\ & \approx 3.21840\quad (\textrm{5 d.p.}) \end{aligned}

14^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}\bigg) \\ & =\frac{15411418072}{5019589575} \\ & \approx 3.07025\quad (\textrm{5 d.p.}) \end{aligned}

15^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}\bigg) \\ & =\frac{467009482388}{145568097675} \\ & \approx 3.20819\quad (\textrm{5 d.p.}) \end{aligned}

16^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}\bigg) \\ & =\frac{13895021563328}{4512611027925} \\ & \approx 3.07915\quad (\textrm{5 d.p.}) \end{aligned}

17^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}\bigg) \\ & =\frac{14442004718228}{4512611027925} \\ & \approx 3.20037\quad (\textrm{5 d.p.}) \end{aligned}

18^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}\bigg) \\ & =\frac{13926277743608}{4512611027925} \\ & \approx 3.08608\quad (\textrm{5 d.p.}) \end{aligned}

19^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}\bigg) \\ & =\frac{533322720625196}{166966608033225} \\ & \approx 3.19419\quad (\textrm{5 d.p.}) \end{aligned}

20^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}\bigg) \\ & =\frac{516197940314096}{166966608033225} \\ & \approx 3.09162\quad (\textrm{5 d.p.}) \end{aligned}

21^{\textrm{st}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}\bigg) \\ & =\frac{21831981985010836}{6845630929362225} \\ & \approx 3.18918\quad (\textrm{5 d.p.}) \end{aligned}

22^{\textrm{nd}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}\bigg) \\ & =\frac{911392701638017048}{294362129962575675} \\ & \approx 3.09616\quad (\textrm{5 d.p.}) \end{aligned}

23^{\textrm{rd}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\bigg) \\ & =\frac{937558224301357108}{294362129962575675} \\ & \approx 3.18505\quad (\textrm{5 d.p.}) \end{aligned}

24^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}\bigg) \\ & =\frac{42887788022313481376}{13835020108241056725} \\ & \approx 3.09994\quad (\textrm{5 d.p.}) \end{aligned}

25^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}\bigg) \\ & =\frac{308120241932332116332}{96845140757687397075} \\ & \approx 3.18158\quad (\textrm{5 d.p.}) \end{aligned}

26^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}\bigg) \\ & =\frac{300524544618003693032}{96845140757687397075} \\ & \approx 3.10315\quad (\textrm{5 d.p.}) \end{aligned}

27^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}\bigg) \\ & =\frac{16315181427784945318996}{5132792460157432044975} \\ & \approx 3.17862\quad (\textrm{5 d.p.}) \end{aligned}

28^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}\bigg) \\ & =\frac{15941887430682586624816}{5132792460157432044975} \\ & \approx 3.10589\quad (\textrm{5 d.p.}) \end{aligned}

29^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}+\frac{1}{57}\bigg) \\ & =\frac{16302083392798897645516}{5132792460157432044975} \\ & \approx 3.17607\quad (\textrm{5 d.p.}) \end{aligned}

30^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}+\frac{1}{57}-\frac{1}{59}\bigg) \\ & =\frac{941291750334505232905544}{302834755149288490653525} \\ & \approx 3.10827\quad (\textrm{5 d.p.}) \end{aligned}

31^{\textrm{st}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}+\frac{1}{57}-\frac{1}{59}+\frac{1}{61}\bigg) \\ & =\frac{58630135791001973169852284}{18472920064106597929865025} \\ & \approx 3.17384\quad (\textrm{5 d.p.}) \end{aligned}

32^{\textrm{nd}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}+\frac{1}{57}-\frac{1}{59}+\frac{1}{61}-\frac{1}{63}\bigg) \\ & =\frac{57457251977407903460019584}{18472920064106597929865025} \\ & \approx 3.11035\quad (\textrm{5 d.p.}) \end{aligned}

33^{\textrm{rd}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}+\frac{1}{57}-\frac{1}{59}+\frac{1}{61}-\frac{1}{63}+\frac{1}{65}\bigg) \\ & =\frac{4507234389098153984166548}{1420993851085122917681925} \\ & \approx 3.17189\quad (\textrm{5 d.p.}) \end{aligned}

34^{\textrm{th}} approximation:

\begin{aligned} \pi & \approx 4\bigg( 1-\frac{1}{3}+\frac{1}{5}-\frac{1}{7}+\frac{1}{9}-\frac{1}{11}+\frac{1}{13}-\frac{1}{15}+\frac{1}{17}-\frac{1}{19}+\frac{1}{21}-\frac{1}{23} \\ & \qquad\quad +\frac{1}{25}-\frac{1}{27}+\frac{1}{29}-\frac{1}{31}+\frac{1}{33}-\frac{1}{35}+\frac{1}{37}-\frac{1}{39}+\frac{1}{41}-\frac{1}{43}+\frac{1}{45}\\ & \qquad\qquad\quad -\frac{1}{47}+\frac{1}{49}-\frac{1}{51}+\frac{1}{53}-\frac{1}{55}+\frac{1}{57}-\frac{1}{59}+\frac{1}{61}-\frac{1}{63}+\frac{1}{65}-\frac{1}{67}\bigg) \\ & =\frac{296300728665235825268431016}{95206588022703235484688975} \\ & \approx 3.11219\quad (\textrm{5 d.p.}) \end{aligned}

Summing without aim, I forgot my purpose. Where am I?

(discontinued)


(refreshed)

Please scroll up to the 7^{\textrm{th}} and 8^{\textrm{th}} approximation.

This required seven or eight terms in the sum for having 3 as the first digit before the decimal point.

202110091141 Exercises 1.1.A (Q1-Q4)

For Exercises 1-4, suppose that an object moves in a straight line such that its position s after time t is the given function s=s(t). Find the instantaneous velocity of the object at a general time t\ge 0. You should mimic the earlier example for the instantaneous velocity when s=-16t^2+100.

1. s=t^2

2. s=9.8t^2

3. s=-16t^2+2t

4. s=t^3


Ans.

1. 2t

2. 19.6t

3. -32t+2

4. 3t^2


Solution.

1.

The average velocity of the object over the interval [t,t+\Delta t] is \frac{\Delta s}{\Delta t}, so since s(t)=t^2:

\begin{aligned} \frac{\Delta s}{\Delta t} & = \frac{s(t+\Delta t)-s(t)}{\Delta t} \\ & = \frac{(t+\Delta t)^2 - t^2}{\Delta t} \\ & = \frac{(t^2+2t\Delta t+(\Delta t)^2)-(t^2)}{\Delta t} \\ & = \frac{2t\Delta t+(\Delta t)^2}{\Delta t} \\ & = \frac{\Delta t(2t+\Delta t)}{\Delta t} \\ & = 2t + \Delta t \end{aligned}

Now let the interval [t,t+\Delta t] get smaller and smaller indefinitely—that is let \Delta t get closer and closer to 0. Then the average velocity \frac{\Delta s}{\Delta t}=2t+\Delta t gets closer and closer to 2t+0=2t. Thus, the object has instantaneous velocity 2t at time t. This calculation can be interpreted as taking the limit of \frac{\Delta s}{\Delta t} as \Delta t approaches 0, written as follows:

\begin{aligned} & \qquad \textrm{instantaneous velocity at }t \\ & = \textrm{limit of average velocity over }[t,t+\Delta t]\textrm{ as }\Delta t\textrm{ approaches to }0 \\ & = \lim_{\Delta t\to 0}\frac{\Delta s}{\Delta t} \\ & = \lim_{\Delta t\to 0}(2t+\Delta t) \\ & = 2t+(0) \\ & = 2t \end{aligned}

2.

\begin{aligned} &\qquad \textrm{instantaneous velocity at }t\\ & = \lim_{\Delta t\to 0}\frac{\Delta s}{\Delta t}\\ & = \lim_{\Delta t\to 0}\frac{s(t+\Delta t)-s(t)}{\Delta t} \\ & = \lim_{\Delta t\to 0}\frac{9.8(t+\Delta t)^2 - 9.8t^2}{\Delta t} \\ & = \lim_{\Delta t\to 0}\frac{9.8(t^2+2t(\Delta t)+(\Delta t)^2) - 9.8t^2}{\Delta t} \\ & = \lim_{\Delta t\to 0}\frac{19.6t(\Delta t)+9.8(\Delta t)^2}{\Delta t} \\ & = \lim_{\Delta t\to 0}19.6t+9.8(\Delta t) \\ & = 19.6t+9.8(0) \\ & = 19.6t \end{aligned}

3.

\begin{aligned} &\qquad \textrm{instantaneous velocity at }t\\ & = \lim_{\Delta t\to 0}\frac{\Delta s}{\Delta t}\\ & = \lim_{\Delta t\to 0}\frac{s(t+\Delta t)-s(t)}{\Delta t} \\ & = \lim_{\Delta t\to 0}\frac{\big( -16(t+\Delta t)^2+2(t+\Delta t)\big) - (-16t^2+2t)}{\Delta t} \\ & = \lim_{\Delta t\to 0}\frac{-16(t^2+2t(\Delta t)+(\Delta t)^2)+2(t+\Delta t)+16t^2-2t}{\Delta t} \\ & = \lim_{\Delta t\to 0}\frac{-32t(\Delta t)-16(\Delta t)^2+2(\Delta t)}{\Delta t} \\ & = \lim_{\Delta t\to 0}(-32t-16(\Delta t)+2) \\ & = -32t-16(0)+2 \\ & = -32t+2 \end{aligned}

4.

\begin{aligned} s(t) & = t^3 \\ s(t+\Delta t) & = (t+\Delta t)^3 \\ & = t^3+3t^2(\Delta t)+3t(\Delta t)^2+(\Delta t)^3 \\ s(t+\Delta t)-s(t) & = \big( t^3+3t^2(\Delta t)+3t(\Delta t)^2+(\Delta t)^3 \big) - (t^3) \\ & = 3t^2(\Delta t)+3t(\Delta t)^2+(\Delta t)^3 \\ \frac{s(t+\Delta t)-s(t)}{\Delta t} & = \frac{3t^2(\Delta t)+3t(\Delta t)^2+(\Delta t)^3}{\Delta t} \\ & = 3t^2+3t(\Delta t)+(\Delta t)^2 \\ \lim_{\Delta t\to 0}\frac{s(t+\Delta t)-s(t)}{\Delta t} & = 3t^2+3t(0)+(0)^2 \\ \frac{\mathrm{d}s}{\mathrm{d}t} & = 3t^2\\ \end{aligned}