Positive real numbers x, y satisfy the equations x^2 + y^2 = 1 and x^4 + y^4 = 17/19. Find xy.
Start with this:
(x^2 + y^2 ) 2 = ( x^4 + y^4) + 2 x^2 y^2 now remember x^2 + y^2 = 1 so
1 = 17/19 + 2 x^2 y^2 I think you should be able to finish from here, yah?