Choose the correct answer from the given four options:
If the common difference of an AP is 5 , then what is $ a_{18}-a_{13} $ ?
(A) 5
(B) 20
(C) 25
(D) 30
Given:
The common difference of an AP is $5$
To do:
We have to find $a_{18}-a_{13}$.
Solution:
Common difference $d=5$
Let $a$ be the first term of the A.P.
$\therefore a_{18}=a+( 18-1)5$
$\Rightarrow a_{18}=a+85$
Similarly, $a_{13}=a+( 13-1)5$
$\Rightarrow a_{13}=a+60$
$\therefore a_{18}-a_{13}=a+85-a-60=25$
Thus, $a_{18}-a_{13}=25$.
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