Ta x = 1 vào PT f(x;y) = (5x-2y+5)(x+4y-4) = 0, ta được
f(1;y) = (5 - 2y + 5)(1 + 4y - 4) = 0
<=> (10 - 2y)(4y - 3) = 0
<=> (5 - y)(4y - 3) = 0
<=> \(\left[ \begin{array}{l}5-y=0\\4y-3=0\end{array} \right.\)
<=> \(\left[ \begin{array}{l}y=5\\y=\frac{3}{4}\end{array} \right.\)