Intersect Sphere


$$||x - c||^2 = r^2$$
$$|| e+ tu - c||^2 = r^2$$
$$(w + tu) \cdot (w + tu) = r^2$$



there is two t which are solution of equation
if $t_0 < 0 < t_1, t1$ is solution
else if $0 < t_0 < t_1, t_o$$0 < t_0 < t_1, t$ is solution







