**M4B+**

We can achieve the same thing (as Method 4A) by rotating both of the parallel red lines through an angle u.

This produces an angle u adjacent to the give angle v.

Again, the adjacent angles u and v are equal to the given right-angle [alternate angles].

Thus u + v = 90˚, so v = 90˚ – u.

This produces an angle u adjacent to the give angle v.

Again, the adjacent angles u and v are equal to the given right-angle [alternate angles].

Thus u + v = 90˚, so v = 90˚ – u.