Ta có : $\widehat{A}=\widehat{B}+\widehat{C}$ và $\widehat{A}+\widehat{B}+\widehat{C}=180^{\circ}$ suy ra $\widehat{A}=\widehat{B}+\widehat{C}=90^{\circ}$
Tam giác IBC có: $\widehat{BIC}=180^{\circ}-(\frac{\widehat{B}}{2}+\frac{\widehat{C}}{2})=180^{\circ}-45^{\circ}=135^{\circ}$
- Đáp án: D