The question is
In a isosceles $\Delta ABC$ $AB=AC=b$ ,$BC=a$ and $\angle BAC=20°$ . Find the relation between $a and b$
I used this $\frac{1}{2} \cdot b^2 sin20°= \frac{1}{2} \cdot a \cdot √(b^2-a^2/4).$ But then I have to find $sin20°$ which I am unable to do so.Please help out with the correct approach.(This is a subjective question with calculators are not allowed) You are allowed to use geometry and trignometry.