Investigate the included angle of straight lines on different planes on one plane.

For example, l 1, l2 is different.

Then let's take a bit from l 1 and make L3∨L2.

Then l 1, l3 realizes * * * plane, and the included angle between them is the included angle of l 1, l2 straight line in different planes.

So naturally, 60 degrees and 90 degrees mean that after moving past, "I found that the angle between l3 and l 1 is 60 degrees and 90 degrees" ... I got the answer.

There is no doubt that the pink line in the picture is different from the grass green line.

But we can cross the grass green line and make the red line parallel to the pink line.

As a result, we found that the red and grass green lines are vertical, so the grass green line is of course vertical to the pink line, which is 90 degrees.