First, let's picture the problem:
Let's label the unknown vertex angle as x
Remember that the sum of angles in a triangle is 180
Hence,
[tex]\begin{gathered} 45^0+45^0+x=180^0 \\ x=180^0\text{ - }90^0 \\ =90^0 \end{gathered}[/tex]
For an isosceles triangle, the base angles are equal
