Question

obtain the sum of first fifteenth term of an A.P. whose first term is 10 and common difference is -7​

Answer 1

Answer:

sn = - 585

Step-by-step explanation:

n= 15

a = 10

d = -7

an = a + (n-1)d

an = 10 + (15 -1 )-7

an = 10 + (-98)

an = - 88

sn = n/2(a + an )

sn = 15/2 ( 10 - 88)

sn = 15/ 2( - 78)

sn = - 585

Answer 2

Sum of first 15 terms=15/2(2*10+(15-1)(-7)
=15/2(20-14*7)=15/2(20-98)=15/2(-78)
=15(-39)
=-585