Question

pls solve question no.21

Answer 1

This can be done using... A.P.

Now...

500/7=71 +4

but 504/7=72

So 504 is the first term. =a

Now.
900/7=142+6
896/7=128

So 896 is the last term An.

here d=7

Now using An=a +(n-1)d
→896=504+(n-1)7
→896-504=(n-1)7
→392=(n-1)7
→n-1=392/7
→n=56+1
→n=57
So there are total 57 terms..

By using Sn=n/2 [ a+An]
→Sn=57/2 [ 504+896]
→Sn=(57×1400)/2
→Sn=57×700=39900

So 39900 is your Required answer...
Hope it helps,
Regards,
Leukonov.