How many terms of the AP : 24, 21,18,... must be taken so that their sum is 78?

Explanation

a=24 and d=21-24=-3

`S_n=n/2(2a+(n-1)d)`

`78=n/2(2(24)+(n-1)(-3))`

156=n(48-3n+3)

`n^2-51n+156=0`

n(n-13)-4(n-13)=0

(n-4)(n-13)

n=4,13

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