Question:
If the common difference of an $\mathrm{AP}$ is 5 , then what is $\mathrm{a}_{18}-\mathrm{a}_{13}$ ?
(a) 5
(b) 20
(c) 25
(d) 30
Solution:
(c) Given, the common difference of AP i.e., d = 5
Now, $\quad a_{18}-a_{13}=a+(18-1) d-[a+(13-1) d] \quad\left[\because a_{n}=a+(n-1) d\right]$
$=a+17 \times 5-a-12 \times 5$
$=85-60=25$