SOLVED! Thank-you jhicks and Tedjn! I am taking a basic calculus course and have some weaknesses when it comes to trigonometry. In this case, it's pure trig. 1. The problem statement, all variables and given/known data The question states: "Find all values of x in the interval [0,2pi] that satisfy the equation sin x = tan x". 2. Relevant equations I do know that: tan x = sin x / cos x 3. The attempt at a solution Since I know that tan x = sin x / cos x, I can rewrite the above equation sin x = tan x to: sin x = sin x / cos x Rearranging and canceling terms, I get: cos x = 1 So my answer to this problem would be x = 0 and 2pi (in radians, of course), that satisfy the given interval. I based this answer on the "Trig Functions of Important Angles" and worked out the multiples of pi that satisfied the equation cos x = 1. My problem is (lol, isn't it always) with the answer key. While they agree that 0 and 2pi are correct, they also add pi. Why? When I consider the cosine of pi (in radians), I get -1, not one. Pi doesn't seem to agree with cos x = 1 when x=pi. Yet, when I work out sin x = tan x, using x=pi, it works out to 0=0, which is certainly true. How did I miss it? Where'd I go wrong?