Click Line
on the Sketch toolbar, or click Tools,
Sketch Entities, Line.
In the Insert Line PropertyManager:
You can create lines of infinite length. This option is available in the Insert Line PropertyManager, regardless of line orientation. Capabilities include:
Trim lines of infinite length when they intersect other sketch entities using the Trim Entities tool.
Add relations such as Use Edge.
To create lines of infinite length:
Click Line
on the Sketch toolbar, or click Tools,
Sketch Entities, Line.
In the Insert Line PropertyManager:
Under Orientation, select an orientation.
Under Options, select Infinite length.
Under Parameters,
set a value for Angle 
.
Only the Angle
orientation includes Parameters.
Sketch one or more lines.
The Line Properties PropertyManager appears once you sketch the first line. You can continue sketching lines with the same Orientation, Options, or Parameters.
To change Orientation,
Options, or Parameters,
click OK 
.
The Insert Line PropertyManager returns, where you can change Orientation or Options, or define Parameters, for Angle orientation.
If you sketched a line of
finite length, to convert the line to infinite length, under Options,
select Infinite length in the
Insert Line PropertyManager. Clear
Infinite length to convert the
line back to finite length. If the line was created with infinite length,
you cannot convert it to a finite length.
When you are finished sketching, click OK
in the Insert Line
PropertyManager and close the sketch.