Originally Posted by
Jhevon ok, we have:
$\displaystyle x - 2y = 10$ .................(1)
$\displaystyle x^2 + y^2 = 20$ ...........(2)
From (1), $\displaystyle x = 10 + 2y$, plug this into (2), we get:
$\displaystyle (10 + 2y)^2 + y^2 = 20$
$\displaystyle \Rightarrow 100 + 40y + 4y^2 + y^2 = 20$
$\displaystyle \Rightarrow 5y^2 + 40y + 80 = 0$
$\displaystyle \Rightarrow y^2 + 8y + 16 = 0$
this is a perfect square, we don't even need the quadratic formula here
$\displaystyle \Rightarrow (y + 4)^2 = 0$
$\displaystyle \Rightarrow y = -4$
now continue