Question

Find the sum of first 20 terms of AP 3.7. 11. 15... using the formula
10​

Answer 1

In the given AP

a=3 and d=4

Sum of 20 terms

I.e. n=20

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

= 20/2(2×3+(19)4)

=10(6+76)

=10×82

=820

I hope this may help u

Answer 2

answer is 820

n=20

a=3

D=4

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

=10(3*2+(20-1) *4)

=10*82

=820