Find all pairs (x,y) of real numbers such that x + y = 10 and x^2 - y^2 = 56.
7.8 and y = 2.2
x + y = 10 ⇒ y = 10 - x
So
x^2 - (10- x ) ^2 = 56
x^2 - x^2 + 20x -100 = 56
20x = 156
x = 156 / 20 = 7.8
y = 10 - 7.8 = 2.2
