Check the picture below.
in the picture below, the bottom part
notice, 69° + 21° = 90°, so 21° and 69° are really complementary angles, sharing the same quadrant, meaning for any two sides of lengths say, a,b one angle will have a tangent of say b/a, then the other will have a tangent of a/b.
namely the tangent of each angle, is simply the other's tangent upside-down.
