Question

Tne sequence 10, 12.5 , 15 , 17.5 ...... Forms an A.P. How many terms of this sequence are required for optaining the sum as 325 ?

Answer 1

Answer:

The difference between between any two consecutive term is constant

d=2.5

Sum of n terms =n/2[(2a+(n-1)d]=325

n/2[20+(n-1)2.5]=325.

n[20+(n-1)2.5]=650

n[40+5n-5]=1300

n[8+n-1]=260

n^2 +7n-260=0

(n-13)(n+20)=0

Therefore n=13