Question

find an ap whose sum of first 3 terms is 21 and the sum of their squares is 155.

Answer 1

let a-d a , and a+d are the three terms of an AP .

now,
a/c to question ,

a -d + a + d = 21

a = 7

again ,

(a - d)² +a² + (a +d)² = 155

3a² -2d² = 155

3(7)² -2d² = 155

147 -2d² = 155

2d² = 8

d² = 4

d = ± 2

so , numbers are ------

7-2 , 7 , 7+2 e.g 5 , 7 , 9

or 9 , 7 , 5