Hello,

There are many x's and y's in the equation of the line. Let be a point of the astroid. The equation of the tangent line at is

Let and be the intersection point of the tangent line with the y-axis and the x-axis, respectively. satisfiesBut how do i show that the length of this line passing through (q,0) and (0,p) is constant?

and is such that

You can now solve these two equations for and and compute to check that this length is constant. (remember that we have )