Lời giải:
a)
$x(a-b)-x(a+b)=xa-xb-xa-xb=-2xb$
b)
$(a^2-ax+by)-(by-a^2-ax)=a^2-ax+by-by+a^2+ax=2a^2$
c)
$(a-b)(a+b)-(b-a)b=a^2-b^2-(b^2-ab)=a^2-b^2-b^2+ab=a^2-2b^2+ab$
\(a,x\left(a-b\right)-x\left(a+b\right)\)
\(=ax-bx-ax-bx=-2bx\)
\(b,\left(a^2-ax+by\right)-\left(by-a^2-ax\right)\)
\(=a^2-ax+by-by+a^2+ax=2a^2\)
\(c,\left(a-b\right)\left(a+b\right)-\left(b-a\right).b\)
\(=a^2-b^2-b^2+ab\)
\(=\left(a-b\right)^2-b^2=\left(a-b-b\right)\left(a-b+b\right)\)
\(=\left(a-2b\right).a\)