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 :)
