Equation of the pair of tangents from a point to a hyperbola

Hyperbola

x²/a² – y²/b² = 1

point (x1,y1)


[x²/a² – y²/b² - 1][x1²/a² – y1²/b² - 1] = [xx1/a² – yy1/b² - 1]²

can be written as SS1 = T²