find the sum of first n terms of an ap whose nth term is 5-6n

 Solution:- Given nth term of an Ap is 5-6n

Means, 

an=5-6n

We know that

d=an-an-1

d=5-6n-{5-6(n-1)}

d=5-6n-{5-6n+6}

d=5-6n-{11-6n}

d=5-6n-11+6n

d=5-11

d=-6

an=5-6n

So, a1=5-6×1

a1=5-6

a1=-1

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

Sn=n/2{2×(-1)+(n-1)-6}

Sn=n/2{2×(-1)+(n-1)-6}

Sn=n/2×(-2-6n+6) 

Sn=n/2×(4+6n) 

Hence sum of nth term of ap Sn=2n^2+3n