Question

If the sum of 1st 7 terms of an AP is 49 and that of 17 terms is 289 find the sum of 1st n terms

Answer 1

Sum of first 7 terms =n/2(2a+(n-1)d)
49×2=7(2a+(n-1)d)
98=14a+7dn-7d -------(I)
Sum of 17 terms =n/2(2a+(n-1)d)
289×2=17(2a+(n-1)d)
578=34a+17dn-17d -------(2)
Now , you can do yourself by multiplying equation (2)by common factor of 17 and 34.then subtract both the equation and then automatically you get the value of a and d .
Then find the sum of 1st term by putting the value a and d in the equation.

Hope this answer will help you!;-)