Question

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

Answer 1

78=n/2[2×24+(n-1)×(-3)],

156=n[48-3(n-1)],

52=n[16-n+1],

52=n(17-n),

52=17n-n²,

then

n²-17n+52=0,


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

n=13 and n=4

Answer 2

you can see your answer