BITSAT 2019 Paper

(d) Equation of pair of tangents is given by
SS‌1 = T‌2,
or S = x‌2 + y‌2 + 20(x + y) + 20,S‌1 = 20,
T = 10(x + y) + 20 = 0
∴ SS‌1 = T‌2
⇒ 20(x‌2 + y‌2 + 20(x + y) + 20) = 10‌2(x + y+2)‌2
⇒ 4x‌2 + 4y‌2+10xy=0
⇒ 2x‌2 + 2y‌2 + 5xy=0