Question

The Sum of first seven terms of an AP is 49 and the sum of first seventeen terms is 289 find the sum of first 20 terms​

Answer 1

400 is right ^^^^^ it should be

Answer 2

S7=49

7/2(2a+6d)=49

2a+6d=49×2/7

2a+6d=14

2a=14-6d

a=7-3d........1

S17=289

17/2×(2a+16d)=289

2a+16d=289×2/17

2a+16d=34

a+8d=17

substituting value of a

7-3d+8d=17

5d=17-7

d=10/5=2

substituting value of d in equation 1

a=7-3×2=1

now

S20=20/2(2a+19d)=10(2×1+19×2)=10(40)

=40×10=400