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!
