a. Vì a//b nên $\widehat{M_{1}}=\widehat{N_{1}}; \widehat{M_{4}}=\widehat{N_{4}}$ (hai góc đồng vị)
Mà $\widehat{N_{3}}=\widehat{N_{1}}; \widehat{N_{4}}=\widehat{N_{2}}$ (hai góc đối đỉnh)
Nên $\widehat{M_{1}}=\widehat{N_{3}}; \widehat{M_{4}}=\widehat{N_{2}}$
b. Vì a//b nên $\widehat{M_{2}}=\widehat{N_{2}}; \widehat{M_{3}}=\widehat{N_{3}}$ (hai góc đồng vị)
Mà $\widehat{N_{1}}+\widehat{N_{2}}=180^{0}; \widehat{N_{3}}+\widehat{N_{4}}=180^{0}$ (Hai góc đồng vị)
Nên: $\widehat{M_{2}}+\widehat{N_{1}}=180^{0}; \widehat{M_{3}}+\widehat{N_{4}}=180^{0}$