Because ABC is the isosceles triangles

We have: \(\widehat{A}\) = 90^o

 * If \(\widehat{A}\) is a corner side => \(\widehat{A}=\widehat{B}or\widehat{C}=90^o\)

=> Last corner = 0^o [nosatisfied]

 * If \(\Delta ABC\) isosceles in A

=> \(\widehat{B}=\widehat{C}=\dfrac{180^0-90^0}{2}=45^0\) [satisfied]

So \(\widehat{B}=\widehat{C}=45^0\)