Question

If the sum of first 7 terms of an A.P is 49 and that of 17 terms is 289. Find the sum of n terms

Answer 1

Let the first term be a &

Common difference be d

S7=49

N/2(2a(n-1)d)=49

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

7/2*2(a+3d). Take 2 common in inside

a+3d=49/7.

a+3d=7

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

S17=289

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

17/2(2a+6d)=289

17/2*2(a+8d). Take 2 common

a+8d=289/17

a+8d=17

7-3d+8d=17

7+5d=17

5d=17-7=10

D=10/5=2

Put value in eq 1

A=7-3(2)

=7-6=1

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

n/2(2+(n-1)2)

n/2(2+2n-2)

n/2(2n). Cancel +2&-2

N square.

Answer 2

Refers to attachment ♡♡~