I have the sine of a large, ugly argument which I know will simplify, e.g. $ $ \sin \left(\frac{2 \left(-c_2 \theta _1-2 c_3 \theta _1-3 c_4 \theta _1+c_2 \theta _2+c_3 \theta _2+c_4 \theta _2+c_3 \theta _3+c_4 \theta _3+c_4 \theta _4-\theta _1\right)}{c_1+2 c_2+3 c_3+4 c_4}-\frac{c_1 \theta _1-c_3 \theta _1-2 c_4 \theta _1-c_1 \theta _2-c_3 \theta _2-2 c_4 \theta _2+2 c_3 \theta _3+2 c_4 \theta _3+2 c_4 \theta _4-2 \theta _1}{c_1+2 c_2+3 c_3+4 c_4}\right) =-\sin \left(\theta _1-\theta _2\right) $ $ The Simplify command works well in this trivial case, but I am dealing with sums of such expressions. Simplify never finishes evaluating in the general case. Is there a way to tell such an expression to Simplify “from the inside out”?