1/ $3x^2-x=3x.x-x=x(x-1)$
2/ $(x-3)^2=x^2-2.x.3+3^2=x^2-6x+9$
3/ $(x-2)(x+1)=x(x+1)-2(x+1)$
$=x^2+x-2x-2=x^2-x-2$
4/ Thay$x=3$: $(-3)^2-49=9-49=-40$
5/ $x^2-9x=0$
$\to x(x-9)=0$
$\to x=0$ or $x-9=0$
$\to x=0$ or $x=9$
6/ $\widehat{A}+\widehat{B}+\widehat{C}+\widehat{D}=360^\circ$
$\to 210^\circ +\widehat{C}+\widehat{D}=360^\circ$
$\to \widehat{C}+\widehat{D}=360^\circ-210^\circ=150^\circ$
7/ Theo tính chất hình bình hành: $C$
8/ $\Delta{ABC}$ đối xứng $\Delta{A'B'C'}$
$\to \Delta{ABC}=\Delta{A'B'C'}$
$\to P_{\Delta{ABC}}=P_{\Delta{A'B'C'}}$
$\to P_{\Delta{ABC}}=32cm$
