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.
Question 9
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? 
Solution
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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