Kẻ Ox//MN. Mà MN//PQ nên Ox//PQ
Vì Ox//MN
Nên: $\widehat{M}$$_{1}=$ $\widehat{O}$$_{1}$ (2 góc so le trong) $\Rightarrow$$\widehat{O}$$_{1}=$ $\widehat{M}$$_{1}=$ $40^\circ$
Vì Ox//PQ
Nên: $\widehat{O}$$_{1}$ $+$ $\widehat{P}$$_{1}=$ $180^\circ$ (2 góc trong cùng phía) $\Rightarrow$$\widehat{O}$$_{1}$ $=180^\circ$$-$ $\widehat{P}$$_{1}$ $\Rightarrow$$\widehat{O}$$_{1}$ $=180^\circ$$-$ $130^\circ=$ $50^\circ$
Mà: $\widehat{MOP}=$ $\widehat{O}$$_{1}$ $+$ $\widehat{O}$$_{2}$
$\Rightarrow$ $\widehat{MOP}=$ $40^\circ$$+$ $50^\circ=$ $90^\circ$
