Question

Find the sum of the following series? -64, -66, -68,......-100

Answer 1

a = - 64

d = - 2

L = - 100

=> a + (n-1)d = - 100

=> - 64 + (n-1) (-2) = - 100

=> (n-1)(-2) = - 36

=> n-1 = 18

=> n = 19


S19 = 19/2 [ a +l]

= 19/2 [ - 64 - 100]

= 19*( - 164) / 2

= 19 (-82)

= - 1558

Answer 2

S=n/2{2a+(n-1)d}

S=n/2{2×(-64)+(n-1)×(-2)

=n/2{-128-2n+2}

=n/2{-126-2n}

=-63n-n^2