(x+y)²=x²+2xy+y²⇔4²=12+2xy⇔16=12+2xy⇔2xy=16-12⇔2xy=4⇔xy=4:2⇔xy=2
(x-y)²=x²-2xy+y²=12-2·2=12-4=8⇒x-y=√8=2√2 sau x-y=-√8=-2√2
