Question

Find the sum of the first 15th tern of an AP 7, 11, 15, 19,.... Formula is xn=f+(n-1)d

Answer 1

Hii friend,

AP = 7,11,15,19,.....

Here,

First term (A) = 7

Common difference (D) = 4

Sn = N/2× [2A+ (n-1) × D]

S15 = 15/2 × [ 2 × 7 + (15-1) × 4]

=> 15/2 × (14 + 56)

=> 15/2 × 70

=> 15 × 35

=> 525

Hence,

The sum of first 15th term of the given AP is 525.

HOPE IT WILL HELP YOU..... :-)

Answer 2

sn=n/2(2a+(n-1)d)
=15/2(2*7+(15-1)4)
15/2(14+56)
15/2(70)
35*15
525


this is ur answer bro