This approximation is a combination (using averages) of the small angle sin(x) = x approximation and another approximation I made using a formula for chord length.
3
Expression 4: "y" equals sine "x"y=sinx
4
Expression 5: "y" equals StartFraction, "x" plus "x" StartRoot, 1 minus StartNestedFraction, "x" squared NestedOver 2 , EndNestedFraction , EndRoot Over 2 , EndFractiony=x+x1−x222