Răspuns
Explicație pas cu pas:
(3/1*4 + 3/4*7 + ... + 3/77*80) = 1/3(3/1 - 3/4 + 3/4 - 3/7 + ... +3/77 - 3/80) =
=1/3(3/1 - 3/80) = 1/3 * 237/80 = 79/80
(2/1*3 + 2/3*5 + ... + 2/73*75) = 1/2(2/1 - 2/3 + 2/3 - 2/5 +... + 2/73 - 2/75 =
=1/2(2/1-2/75) = 1/2 * 148/75 = 74/75
x^2 = 3 * 79/80 - 74/75 = 237/80 - 74/75
=> x^2 = 2371/1200 => x = sqrt(2371/1200)
