Another doodle.
Here we have (somehow) hit upon drawing a completely new line, which again produces a triangle.
We know one of its angles is 90˚ [given] so the sum of the other two is 90˚. We don’t know their specific values but we can write them as, say, e and 90˚– e.
Thus u + e + 90˚– e + v = 180˚ [‘supplementary angles’ property],
so u + v = 90˚, so v = 90˚ – u.