In an isosceles triangle of angles ABC there is one angle C that is equal to 0.68.I subtract 0.68 from 1 to obtain a result of 0.32.By using the following equation$\frac{1}{\sqrt(1\times((1-0.68)\times2))} =1.25$
To verify I take half of the the base that is equal to to 0.5 , divided by 1.25 to obtain 0.4,for angle A and 0.4 for angle B, 0.68 for Angle C. $0.4^2\times2=0.32$.
Do you only need to know one angle only for equilateral and isosceles triangles?
No, you need more than that. With just one angle given you can have infinitely many {isosceles/equilateral} triangles. Try drawing things out.