Question

(ii) The sum of the 3rd and 7th terms of an A.P. is 32 and their product is 220. Find
the sum of first twenty-one terms of the A.P. ​

Answer 1

Step-by-step explanation:

a+2d+a+6d= 32 == 2a+28d= 32 = a+14d=16 --(1)

(a+2d)(a+6d)= 220 ------------------------(2)

S21= 21/2(2a+20d)

Solving (1) and (2) to get a=4 and d=3

So S21= 21(34)==714