作为微积分中两个重要函数极限之一的是三角函数求导的基础(它本身就是sinx,在x=0处的导数)。正因这样,每一本微积分教科书都少不了要给它一个证明。证明方法大同小异。都是从下面的图形(图1)出发,其中x为用弧度度量的半径为1的圆心角。AD为过弧x的端点的关于圆的切线。显 As one of the two important function limits in calculus is the basis for the derivation of a trigonometric function (itself is sinx, the derivative at x=0). Because of this, every textbook of calculus will have to give it a proof. The method of proof is much the same. Both start from the following graph (Figure 1), where x is the central angle of radius 1 measured in radians. AD is the tangent to the circle of the endpoint of over arc x. Obvious