Question

Find S11 for 5 + 1 + (-3) + (-7) +…

Answer 1

The answer for this is -165

Answer 2

Known :

a = 5

b = U2 - U1

= 1 - 5

= -4

Asked :

S11 = ...?

Answer :

Sn = n/2 (2a + (n-1) b)

S11 = 11/2 (2×5 + (11-1) (-4))

S11 = 11/2 (10 + 10 × (-4))

S11 = 11/2 (10 - 40)

S11 = 11/2 × (-30)

S11 = 11 × (-15)

S11 = -165

Hope it helpful and useful :)