Opgave 1
$\cases{2x+3y=12\cr3x+7y=8}$
$\cases{6x+9y=36\cr6x+14y=16}$
Trek (1) van (2) af:
$5y=-20$
$\cases{y=-4\cr x=12}$
Opgave 2
$\cases{2x+3y=12\cr y=2x+4}$
$\cases{2x+3(2x+4)=12\cr y=2x+4}$
$\cases{2x+6x+12=12\cr y=2x+4}$
$\cases{8x=0\cr y=2x+4}$
$\cases{x=0\cr y=4}$
Opgave 3
$\cases{y=x^2-4x\cr y=2x-4}$
$x^2-4x=2x-4$
$x^2-6x+4=0$
$(x-3)^2-9+4=0$
$(x-3)^2-5=0$
$(x-3)^2=5$
$x-3=-\sqrt 5\vee x-3=\sqrt 5$
$x=3-\sqrt 5\vee x=3+\sqrt 5$
$\cases{x=3-\sqrt 5\cr y=2-2\sqrt 5}\vee \cases{x=3+\sqrt 5\cr y=2+2\sqrt 5}$