Question

If the 5th and 12th terms of an AP are $30$ and $65$ respectively, what is the sum of the first 20 terms?

Answer 1

$a+4d=30$

$a+11d=65$

$\Rightarrow d=5,a=10$

$\therefore$ sum of 20 items

$\frac{20}{2}\left(2\right(10)+(20-1\left)5\right)$

$10(20+95)$

$1150$