Vi C thuoc Elip nen thi C co toa do $(2\sqrt{2}sina; 2cosa) $ dk $a\in [0;2\pi ]$
De dien tich tam giac la lon nhat thi khoang cach tu C den (d) la lon nhat
Co $d(C,AB)=\frac{\left| {2\sqrt{2}sina-2\sqrt{2}cosa+2} \right|}{\sqrt{3}}=\frac{\left| {4sin(a-\frac{\pi }{4})+2} \right|}{\sqrt{3}}$
De khoang cach nay lon nhat thi $sin(a-\frac{\pi }{4})=1<=> a= \frac{3\pi }{4}$
De dang co $C(2;\sqrt{2})$