Solution (source code)

= Solution

At a point of tangency, the <derivative> of the curve equals the <slope> one of the straight line. The <chain rule> gives $2\cos2x=1$. Since $0\le2x\le\pi$, the unique solution is $x=\pi/6$. The corresponding height is $\sin(\pi/3)=\sqrt3/2$. Substitution in the <tangent line> gives
$$
\boxed{c=\frac{\sqrt3}{2}-\frac\pi6.}
$$