Question

how many terms of an AP:24, 21,18... must be taken so that their sum is 78

Answer 1

a = 24
d = -3
Sum of AP = n/2(2a + (n-1)d)
Therefore

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

n( 48 - 3n + 3) = 156

51n - 3n^2 = 156

Multiplying whole equation by -1

3n^2 -51n +156 = 0

Dividing whole equation by 3

n^2 -17n +52=0

Therefore n = 4

HOPE IT HELPED
THANKS...