Ask a Teacher

Search



In an AP 5th and 12th terms are 30 and 65 respectively. What is the sum of first 20 terms

 We have
an=a+(n-1)d
Given
a5=30
a+4d=30 ...(1)

and a12=65
a+11d=65...(2)

(2) - (1),
7d=35
d=5
Now from (1),
a+20=30
a=10

For sum of n terms we have formula
Sn=n / 2[2a+(n?1)d];   Put n=20, a=10 and d=5;    

S20=20/2[2*10+(20?1)5]
=10[20+95]
=1150



comments powered by Disqus