1. The problem statement, all variables and given/known data A rod lies at an angle α with the x'-axis of an inertial frame moving at a speed v along the x-axis(x and x' are parallel) of another inertial frame. The rod makes angle β with the x-axis of this frame. Find the relation between α and β. Variables: α,β,v and define γ=1/√1-(v/c)2; x'=projection of rod along x.-axis with respect to first inertial frame; z'=projection of rod along z'-axis(perpendicular to x') with respect to first inertial frame; x=projection of rod along x-axis with respect to second inertial frame; z=projection of rod along z-axis with respect to second inertial frame; 2. Relevant equations Since lengths perpendicular to the relative motion remains unchanged, z'=z; According to the second inertial frame, the length of the rod along the direction of motion is contracted by a factor of γ. Thus, x=(1/γ)x'. tanα=z'/x' and tanβ=z/x. 3. The attempt at a solution Thus, tanβ=γtanα. The problem is, the answer given in the book is, tanα=γtanβ. I am confused, please help.