Question

how many terms of the ap 3 , 5 , 7 , ...... should be taken to get the sum of 120

Answer 1

Step-by-step explanation:

Given that,

A.p is 3,5,7....

we know that a=3 and d=2

s(n)=120

s(n)=n/2[2a+(n-1)d]=120

n/2[2(3)+(n-1)2]=120

n/2(6+2n-2)=120

n/2(4+2n)=120

n/2[2(2+n)]=120

n(2+n)=120

2n+n^2=120

n^2+2n-120=0

n^2+12n-10n-120=0

n(n-12)-10(n+12)=0

(n-10)(n+12)=0

n=10 or n= -12

therefore,no.of terms =10