Find the sum of n terms of an A.P. whose nth term is given by
Put n = 1
a1 = 5 – 6(1) = 5 – 6 = -1
Put n = 2
a2= 5 – 6(2) = 5 – 12 = -7
a = -1, d = -7 + 1 = -6
Sn = 
[2a + (n – 1) d]
= 
[2(-1) + (n -1) (-6)]
= 
[-1 + 3n + 3]
= n [2 -3n]
3
Find the sum of n terms of an A.P. whose nth term is given by
Put n = 1
a1 = 5 – 6(1) = 5 – 6 = -1
Put n = 2
a2= 5 – 6(2) = 5 – 12 = -7
a = -1, d = -7 + 1 = -6
Sn = [2a + (n – 1) d]
= [2(-1) + (n -1) (-6)]
= [-1 + 3n + 3]
= n [2 -3n]