a, \(\widehat{BEC}\) = \(\widehat{BAC}\) + \(\widehat{ABE}\) ( tính chất góc ngoài)
hay \(\widehat{BEC}\) = 90\(^{\circ}\) + \(\widehat{ABE}\)
=> \(\widehat{BEC}\) > 90\(^{\circ}\)
=> \(\widehat{BEC}\) là góc tù
b, \(\widehat{BAC}\) = 90\(^{\circ}\)
=> \(\widehat{ABC}\) + \(\widehat{ACB}\) = 90\(^{\circ}\)
Lại có : \(\widehat{ACB}\) - \(\widehat{ABC}\) = 20\(^{\circ}\)
=> \(\widehat{ABC}\) = 35\(^{\circ}\)
\(\widehat{ABE}\)=\(\frac{\widehat{ABC}}{2}\)=\(\frac{\widehat{35^{\circ}}}{2}\) = 17,5 \(^{\circ}\)
\(\widehat{AEB}\)=\(\widehat{BAE}\)-\(\widehat{ABE}\) = 90\(^{\circ}\) - 17,5\(^{\circ}\)= 72,5\(^{\circ}\)
\(\widehat{BEC}\)= 180\(^{\circ}\) - \(\widehat{AEB}\) = 180\(^{\circ}\) - 72,5\(^{\circ}\) = 107,5 72,5\(^{\circ}\)
