a) GT: $\widehat{xOy}+\widehat{uHv}=90^{\circ},\widehat{x'O'y'}+\widehat{uHv}=90^{\circ}$.
KL: $\widehat{xOy}=\widehat{x'O'y'}
Chứng minh: $\widehat{xOy}=90^{\circ}-\widehat{uHv}=\widehat{x'O'y'}.$
b) GT: $\widehat{xOy}+\widehat{uHv}=180^{\circ},\widehat{x'O'y'}+\widehat{uHv}=180^{\circ}$.
KL: $\widehat{xOy}=\widehat{x'O'y'}$.
Chứng minh: $\widehat{xOy}=180^{\circ}-\widehat{uHv}=\widehat{x'O'y'}$.