Ta tinh goc giua 2 duong thang $d_{1}$ va $d_{2}$
$cos\alpha =\pm \frac{1}{\sqrt{10}}$
$=>sin\alpha =\frac{3}{\sqrt{10}}$
Ap dung dinh ly ham sin cho tam giac ABI
$\frac{AB}{sin\alpha }=\frac{AI}{sin\beta }=>sin\beta =\frac{1}{\sqrt{10}}$ vs $\beta $ la goc giua $d_{1}$ va $\Delta $
Tu do suy ra $cos\beta =\pm \frac{3}{\sqrt{10}}$
Tiep theo la lap pt duong thang $\Delta $ qua M(-1;1) va tao vs $d_{2}$ goc $\beta $
Bai toan co ban?