Question

What is the sum of the first 11 terms of the arithmetic series in which a1 = 12 and d = 5?

A 38

B 549

C 143

D 209

Answer 1

sum S = (n/2) × (2a1 + (n-1)d)
calculate S by putting n=11

Answer 2

so putting value in the above formula it is cumming 407 and that is not any of the options

Answer 3

if that so options may be wrong.

Answer 4

sum (1 to n) = (1/2)(n)(n+1)

5 sum (1 to n) = (5) (1/2)(n)(n+1) = 5(11)(12)/2 = 330

11 a1= (11)12= 132

I could not get any of the options by brute force 12 + 17 + 22 +...