### 2.20.2 Combine terms of same type : `combine`

`combine` takes two arguments : an expression and the name of a function or class of functions `exp,log,ln, sin,cos,trig`.
Whenever possible, `combine` put together subexpressions corresponding to the second argument:

- `combine(expr,ln)` or `combine(expr,log)` gives the same result as `lncollect(expr)`
- `combine(expr,trig)` or `combine(expr,sin)` or `combine(expr,cos)` gives the same result as `tcollect(expr)`.

Input :

`combine(exp(x)*exp(y)+sin(x)*cos(x)+ln(x)+ln(y),exp)`

Output :

`exp(x+y)+sin(x)*cos(x)+ln(x)+ln(y)`

Input :

`combine(exp(x)*exp(y)+sin(x)*cos(x)+ln(x)+ln(y),trig)`

or

`combine(exp(x)*exp(y)+sin(x)*cos(x)+ln(x)+ln(y),sin)`

or

`combine(exp(x)*exp(y)+sin(x)*cos(x)+ln(x)+ln(y),cos)`

Output :

`exp(y)*exp(x)+(sin(2*x))/2+ln(x)+ln(y)`

Input :

`combine(exp(x)*exp(y)+sin(x)*cos(x)+ln(x)+ln(y),ln)`

or

`combine(exp(x)*exp(y)+sin(x)*cos(x)+ln(x)+ln(y),log)`

Output :

`exp(x)*exp(y)+sin(x)*cos(x)+ln(x*y)`