Question: Two bikers leave a diner at the same time. Biker Slim rides at 85kmh [N] and Biker Haug rides at 120kmh [NE]. How fast is the distance between them changing 40 minutes after they left? I suggest looking at my photos of the triangles and such, as explaining it over text can be a bit confusing. Attempt: http://imgur.com/a/lkyvL [Photos are out of order, sorry.] So I drew out the triangle, and it is not a right angled triangle by nature. But I can break it apart into two right angle triangles. From my photo, I solve for the speed of side X by using Sine Law, and also I found that length X = length (A-Z). From there and knowing (A) from the beginning, and having found (A-Z) I was able to determine Z using simple algebra. After that, and using pythagorean's theorem I was able to find side C with sides Z and X. Once that was done, I took the lengths of X, Z, and C and put them into pythagorean's theorem and derived it, in search of dc/dt or the rate at which the distance between them. Another guy in my class agrees with my answer but my teacher told me it's around 49. My teacher created and solved it himself so there is not really a verification we can do, he also makes lots of mistakes (as everyone does in math) so were are unsure who is right or if we made a flub somewhere. If someone could look over my work and point out/help me realize my mistake it would be appreciated.