Question

How many terms of the A.P. : 2, 7, 12, …must be
taken to give a sum of 245?

Answer 1

Answer:

10 terms and we will get the sum

Answer 2

Answer:

In this AP series 1st term a=2

common difference d=7-2=5

let the number of terms is n

here the sum is 245

so,

n/2{2a+(n-1).d}=245

=> n/2{2.2+(n-1).5}=245

=> n(4+5n-5)=245×2

=> n(5n-1)=490

=> 5n^2-n-490=0

=> 5n^2-50n+49n-490=0

=> 5n(n-10)+49(n-10)=0

=> (n-10)(5n+49)=0

=> n-10=0

=> n=10

again, 5n+49=0

n= -49/5 which is not possible

so number of terms is 10