/ l
/
/
C / B
---------------- m
A / D
/
/
/
/
/
A and B are
vertical angles. C and D are
vertical angles. This is contingent upon
l and
m being straight lines. Note that A and B have the same magnitude, and that C and D have equal magnitudes as well. This is because
A + C = π, A + D = π, D + B = π, B + C = pi;, and in the nice world of
Euclidean geometry, you can equate things that have the same value (A + C = A + D, eg) and then subtract off the common additives (C = D).