1)(x-y)(x+y)=40
10*(x-y)=40
x-y=4
x=4+y
4+y+y=10
2y=6
y=3
x=7
2)(x-y)(x+y)=40
4(x+y)=40
x+y=10
x=10-y
10-y-y=4
-2y=-6
y=3
x=7
3)x=2-y
(2-y)y=-15;2y-y^2=-15;y^2-2y-15=0;y1=5,y2=-3;
x1=-3;x2=5
4)
y=5-x;x(5-x)=-14;x^2-5x-14=0
x1=7;x2=-2;y1=-2;y2=7
5)x=y+1
(1+y)^2-y=3
y^2+y-2=0
y1=-2;y2=1
x1=-1;x2=2
6)x=4-y
(4-y)^2-y=2
y^2-9y+14=0
y1=7;y2=2;
x1=-3;x2=2.