Question

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

• A. $-75$
• B. $-125$
• C. $75$
• D. $125$

Answer 1

Answer: (A)

$-75$

$\begin{array}{c}15,10,\mathrm{5.......}\\ S_{10}=2\\ a=15,d=10-5=-5\\ Sn=\frac{n}{2}[2a+(n-10\left)d\right]\\ S10=\frac{10}{2}[2\times 15+(10-1)\times -5]\\ =5[30-45]\\ =-75\end{array}$