I have a three link revolute manipulator at the origin. I know all the link lengths. The joint angle for the third link is coupled to the second such that Theta3 = k*Theta2. I want to determine the joint angles (thetas) of the manipulator given that the third link should lie on a line an angle Alpha from the x - axis at a known position. I believe the solution should be unique but I can't seem to wrap my hands around an equation that proves it. To summarize: Knowns: Link lengths: L1, L2, L3 Angle of line: Alpha = Theta1 + Theta2 + Theta3 = Theta1 + Theta2(1+k) And it's position Coupling function: Theta3 = Theta2 * k Unknowns Theta1, Theta2, Theta3 Joint positions, X1,Y1, X2, Y2, X3, Y3 I tried assuming I knew one of the joint end positions, i.e X2, Y2, because my intuition tells me there's got to be only one solution. Then maybe I could find another set of equations and somehow cut it out. Here's my work so far: By law of cosines: X2^2 + Y2^2 = L1^2 + L2^2 - 2L1L2cos(beta) where beta is 180 - Theta2 (see diagram) beta = acos((L1^2 + L2^2 - (X2^2 + Y2^2)) / (2L1L2)) Theta2 = 180 - beta Theta3 = (180 - beta) * k Theta1 = alpha - Theta2 - Theta3 But now I'm stuck because I can't come up with another good set of equations to remove X2 and Y2 since I don't actually know them. Any insight or advice would be great. Thanks!