Find the arithmetic progression and sum of its first 20 terms whose arithmetic mean and geometric mean between first and third term is 10 and 8 respectively

Answer 1

(a + a+2d)/2 = 10
√(a(a+2d)) = 8

a=4, d=6

So, the sequence is

4,10,16,...

S20 = 20/2 (2*4 + 19*6) = 1220

Answer 2

Im doing 8th grade statistics, i did 7th grade Geometry and Arithmetics before.