Q. The first term of a GP is 7, the last term is 448 and sum of all term is 889, then the common ratio is
A) 5
B) 4
C) 3
D) 2
Explanation
(d)
Given, $a = 7,\ ar^{n - 1} = 448$ and $S_{n} = \frac{a(r^{n} - 1)}{r - 1} = 889$
$\Rightarrow \frac{ar^{n - 1}r - a}{r - 1} = 889\ \Rightarrow \ \frac{448r - 7}{r - 1} = 889$
$\Rightarrow \ 448r - 7 = 889(r - 1)$
$\Rightarrow$ $r = 2$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.