Question

$15,10,5,\dots \dots \dots$ In this A.P., sum of first 10 terms is

Answer 1

$15,10,5,...\dots .$

$a=15$

$d=10-15=-5$

$S_n=\frac{n}{2}[2a+(n-1\left)d\right]$

$S_{10}=\frac{10}{2}[2\times 15+(10-1\left)\right(-5\left)\right]$

$=5[30-5\times 9]$

$=5[30-45]$

$=5(-15)=-75$.