1. The problem statement, all variables and given/known data My question is part C of problem 1.9: http://facultyfiles.deanza.edu/gems/lunaeduardo/4DHW.PDF 2. Relevant equations Pythagorean Theorem? 3. The attempt at a solution I have c vector of lightwave going upwards and v going rightwards forming a right angle with the other vector. Now, I need to find the observed speed of the lightwave on earth. I would do √(c^2 + v^2), since the particles are being separated by the hypotenuse, but I think it's wrong I have seen some answers online that involve a minus sign for some reason. Could someone elucidate? Thank you.