Question

Find S25 for -7+ -2 + 3 + 8 +…

Answer 1

We know S(n) = n/2 [ 2a + (n-1)d ]
Here, n = 25
a = -7
d = -2-(-7) = -2+7 = 5

Substitute their values, 
S(25) = 25/2 [2(-7) + (25-1)5 ]
S(25) = 25/2 [-14 + 120 ]
S(25) = 25/2 [106 ]
S(25) = 25 [ 53 ]
S(25) = 1325

In short, Your Answer would be 1325

Hope this helps!