Question 508883
Proof:

By definition, if m<2 and m<3 are complementary, then  m<2 + m<3 = 90.  Also since we are given m<1 + m<2 = 90, then it follows that m<1 + m<2 =  m<2 + m<3.  Then if we subtract m<2 from both sides we are given our desired result m<1 = m<3. Hence m<1 and m<3 are congruent.