and this formula is simply an expression of the fact that given a circle: x = sin t , y = cos t the derivative is d(sin t) = (cos t) dt , d(cos t) = -(sin t

and this formula is simply an expression of the fact that given a circle: x = sin t , y = cos t the derivative is d(sin t) = (cos t) dt , d(cos t) = -(sin t) st so it's interesting that it was "stumbled" upon.