Complementary angles
Diagram
![[asy] import olympiad; size(150); pair D = (0, 10), C = (10, 5), B = (10, 0), A = (0, 0); draw(A--B); draw(C--A); draw(D--A); label("$A$", A, SW); label("$B$", B, SE); label("$C$", C, W); label("$D$", D, NE); [/asy]](http://latex.artofproblemsolving.com/1/b/d/1bd23ed11664b0cee697fc4b5c38c234b7f6d6a1.png)
As long as 
is a right angle, 
and 
are complementary angles to each other.
Definition
Bing/Google
Either of two angles whose sum is 90°
Merriam-Webster
Two angles that add up to 90 degrees
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