(h.141)\(\Delta AOM\) cân \(\Rightarrow\)\(\widehat{A}=\widehat{M1}\)
\(\Delta BOM\) cân \(\Rightarrow\)\(\widehat{B}=\widehat{M2}\)
Suy ra \(\widehat{M1}+\widehat{M2}=\widehat{A}+\widehat{B}\)do đó
\(\widehat{AMB}=\widehat{A}+\widehat{B}\).Ta lại có:
\(\widehat{AMB}+\widehat{A}+\widehat{B}=180^0\) nên
\(\widehat{AMB}=90^0\)
(h.141)ΔAOM=>A^=M1^
ΔBOMΔBOM cân ⇒B^=M2^
Suy ra ˆM1+ˆM2=ˆA+ˆBdo đó
ˆAMB=ˆA+ˆB.Ta lại có:
ˆAMB+ˆA+ˆB=180o nên
ˆAMB=90o