Question

How many terms ap 24,21,18..Must be taken si their sum is 78?

Answer 1

Answer:

4 and 13.

Step-by-step explanation:

Sₙ= n/2[2a+(n-1)d]            { a= 24, d= -3}

78= n/2[48+(n-1)(-3)]

n²-17n-52=0 ==> n= 4 , 13.