Question 9 Exercise 4.2

Solutions of Question 9 of Exercise 4.2 of Unit 04: Sequence and Series. This is unit of A Textbook of Mathematics for Grade XI is published by Khyber Pakhtunkhwa Textbook Board (KPTB or KPTBB) Peshawar, Pakistan.

A ball rolling up an incline covered $24m$ during the first second, $21m$ during the second, $18m$ during the third second. Find how many meters it covered in the eight second?

Suppose that $$a_1=24,$$ $$a_2=21,$$ $$a_3=18.$$ Since $$d=21-24=18-21=-3,$$ given sequence is in A.P. Now $$\begin{align} a_8&=a_1+7d\\ &=24+7(-3)=3. \end{align}$$ Hence the distance covered by the ball in 8th second is $3m$.

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