Question

find the sum of first 11 terms which are multiples of 2 as well as of 9

Answer 1

To find the given sum, you need to first find numbers which are multiples of both 2&9.
the numbers will be the multiples of their LCM i.e. 18
so the numbers are 18+36+.......11 terms
here a=18
n=11
d= 18
sum = n/2 [2a +(n-1)d]
= 11/2 [2x18 + (11-1) 18]
= 11/2 [36 + 180]
= 11/2 x 216
= 11 x 108
= 1188

Answer 2

AP = 18, 36, 54 ............. a11
a = 18
d = 18
n = 11
an = 18 + (11-1) (18)
an = 18 + 180
an = 198

S11 = 11/2 (18 + 198)
S11 = 11/2 × 216
S11 = 11 × 108
S11 = 1188