# Envelopes: envelope

### 13.18.2 Envelopes: envelope

The envelope command draws the envelope of a family of curves.

- envelope takes two arguments:
  - expr, an expression of two variables and one parameter.
  - L, a list of the names of the variables and the parameters. If the variables are x and y, then L only need to be the name of the parameter.
- envelope(expr,L) draws the envelope of the family of curves given by expr=0 over the parameter.

#### Examples.

- Input:
  envelope(y + x*tan(t) - 2*sin(t),t)
  Output:
- Input:
  envelope(v + u*tan(s) - 2*sin(s),[u,v,s])
  Output: