J = Matrix([[-l1*sin(q1) - l2*sin(q1 + q2), -l2*sin(q1 + q2)], [l1*cos(q1) + l2*cos(q1 + q2), l2*cos(q1 + q2)]])
det J simplifies to: l1*l2*sin(q2)  (expected l1 l2 sin q2) True
x^2+y^2 = l1**2 + 2*l1*l2*cos(q2) + l2**2   -> cos q2 = (x^2+y^2-l1^2-l2^2)/(2 l1 l2): True
J^-1 = Matrix([[cos(q1 + q2)/(l1*sin(q2)), sin(q1 + q2)/(l1*sin(q2))], [-(l1*cos(q1) + l2*cos(q1 + q2))/(l1*l2*sin(q2)), -(l1*sin(q1) + l2*sin(q1 + q2))/(l1*l2*sin(q2))]])
norm of J^-1 column growth: 1/|sin q2| factor: l2*cos(q1 + q2)
