We need to draw an isosceles triangle △ ABC with m∠ B=90^(∘). This means that △ ABC is a right triangle. From this information, we can say that the congruent sides are the legs of △ ABC, since the hypotenuse has to be the longest side.
We have that AB ≅ BC. By applying the Isosceles Triangle Theorem, we get that ∠ A ≅ ∠ C.
From the above, we have that m∠ A=m∠ C. Since the sum of the measures of the interior angles of a triangle must add to 180^(∘), we get the following equation.
m∠ A+ m∠ B + m∠ C &= 180^(∘)
Next let's substitute m∠ B=90^(∘) and m∠ A=m∠ C, and solve the resulting equation for m∠ A.
