I have an algebraic output (a solution to a de) similar to, but far longer than:


I'd like to get Mathematica to notice that the group x+3 appears repeatedly, define a new parameter par1=x+3, and rewrite the original output as:

{par1=x+3, par1^2/sin(7*par1)+par1^3/7}

The new expression would be far simpler and faster to evaluate than the original if par1 is, say 40 characters long, and appears say 6 times in the original expression.

Ideas on how to accomplish this?