Given: 1 is complementary to 2. 2 is complementary to 3. prove: m1 = m3 what is the missing statement in step 3 of the proof? m1 = m2 m1 + m2 = 90° m2 = m3 m2 + m3 = 180°

First, it has to be noted that the term complementary refers to two angles with measures summing up to 90o.  A two-column proof is shown below.

                                    M1 = m2                                 conclusion

                                    M1 + m2 = 90o                                           Definition of complementary angles

The missing term here is:

                                    M2 + m3 = 90o              Definition of complementary angles

                                    M1 + m2 = m2 + m3             

Subtracting m2 from both sides of the equation,

                                    M1 = m3

The answer is A