Question

how many terms of the AP 54 51
48 Sean must be taken so that their sum is 513

Answer 1

Solve on the this formula ... ½b×h

Answer 2

Given

a = 54

d = -3

Sn = 513

Sn = n/2(2a + (n-1)d)

513 = n/2( 108 + 3 -3n)

1026 = 111n - 3n^2

n^2 -37n +342 = 0

on solving u will get

n= 18 , n=19

You are getting here 2 values , both positive so , Both are applicable here.

Since d=negative

A case will come when the no. after 0 will cancel out the no. before 0 ,

Positive no. will cancel negative no.

So ,both the cases are applicable.....

You may try for both taking n=18 or n= 19 , u will get the same answer..