Probier das mal wie folgt
a·x + b·y = a^2 + b^2
- b·x + a·y = a^2 + b^2
I·b, II·a
a·b·x + b^2·y = (a^2 + b^2)·b
- a·b·x + a^2·y = (a^2 + b^2)·a
II + I
a^2·y + b^2·y = (a^2 + b^2)·a + (a^2 + b^2)·b
(a^2 + b^2)·y = (a^2 + b^2)·(a + b)
y = a + b
a·x + b·y = a^2 + b^2
a·x + b·(a + b) = a^2 + b^2
a·x + a·b + b^2 = a^2 + b^2
a·x + a·b = a^2
x + b = a
x = a - b