ta có công thức
$\left \{ {{x'=x.cos\alpha-y.sin\alpha} \atop {y'=x.sin\alpha+y.cos\alpha}} \right.$
=>$\left \{ {{x'=1.cos\dfrac{-\pi}{2}-3.sin\dfrac{-\pi}{2}} \atop {y'=1.sin\dfrac{-\pi}{2}}+3.cos\dfrac{-\pi}{2}} \right.$
<=>$\left \{ {{x'=0+3} \atop {y'=-1+0}} \right.$
<=>$\left \{ {{x'=3} \atop {y'=-1}} \right.$
=>A'(3;-1)
xin hay nhất nhé