The first and last terms of an A.p are 10 and 90,if the sum of terms is 750,the number of terms is used to find the common difference if the sum is 1400?

Answer 1

You are telling me that a = 10 and l = 90
sum of tems = (n/2)(a+l) = 750
(n/2)(10+90) = 750
50n = 750
n = 15

Also we know
a + (n-1)d is the last term
10 + 14d = 90
14d = 80
d = 40/7

"the number of terms is used to find the common difference if the sum is 1400?" --- I don't know what that means, but if sum of n terms is 1400
(n/2)(10+90) = 1400
50n = 1400
n= 28