√(x/y) + √(y/x) = 5/2 | xy > 0
x/y + 2√(x/y)*√(y/x) + y/x = 25/4
x/y + y/x = 25/4 - 2 = 17/4
(x/y)^2 - 17/4*x/y + 1 = 0
x/y = ( 17 +- √(289 - 64) )/8 = {1/4, 4}
x^2 - y^2 = 15
x = 4y -> 15y^2 = 15 -> y = +-1, x = +-4
x = y/4 -> -15x^2 = 15 - нет решений
-->
x = 4, y = 1
x =-4, y =-1