Show that the equation of the tangent to xy=c^2 at (x1, y1) is x1y + xy1= 2c^2
so far i've gotten dy/dx= -c^2/ x^2,
y- y1= (-c^2/x1^2)(x-x1)
y(x1^2)- y1(x1^2)= (-c^2)x + (c^2)x1
and thats as far as i went, from here on i have no idea how to get to the eqn of the tangent, please help?