Yikes! I've need help with two questions that seem to involve hard and tedious trig: 1. How do I prove that an equilateral triangle can be inscribed in an epitrochoid? 2. How do I find the coordinates of a centroid of an equilateral triangle (given the x- and y- coordinates of its three vertices)? (I have no idea where to get started) P.S. Can anyone tell me how to find the sum of a harmonic series with all the numbers that have the digit zero removed? (E.g., (1/1 + ... 1/9 ) + (1/11 + ... 1/19)). I've gathered that I'm supposed to group the numbers as (1/1 + ... 1/9) + (1/11 + ... 1/99) + (1/111 + 1/999) + ..., but I don't know what to do from there on.