find the sum of first n terms of an ap whose nth term is 5n-1

 Solution

Given that An=5n-1

Then d=an-an-1

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

d=5n-1-{5n-5-1}

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

d=5n-1-5n+6

d=+5

Also An=5n-1

a+(n-1)d=5n-1

a+(n-1)×5=5n-1

a+5n-5=5n-1

a=5-1

a=4

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

           Sn=n/2{2×4+(n-1)5}

           Sn=n/2{8+5n-5}

           Sn=n/2(3+5n) ans