Question

the first term of an A.P. is 1 and the sum of first 15 terms is 225 find the common differences

Answer 1

first term of an AP is 1
so,a=1

and, Sum of 15 terms = 225

S15 = 225

15/2(2a+ (15-1)d)= 225

15/2(2+14d)=225

2+(14)d = 30

14d= 28
d= 2


Hence,common difference is 2

Answer 2

Step-by-step explanation:

a=1

s15=225

s15=(n/2)(2a+(n-1)d)

225=(15/2)(2(1)+(15-1)d)

225=(15/2)(2+14d)

225*2/15=2+14d

14d+2=450/15

14d+2=30

14d = 30-2

d=28/14

d=2