想要请教一个问题，
在做杰里和瑞尼那本高微的课后习题，第三章theory of the firm的课后习题3.2

let y=f(x1, x2) be a constant returns to scale production function. show that if the average product of x1 is rising,
the marginal product of x2 is negative.

下图是习题答案，想请问一下，第二行的式子是怎么得来的。谢谢！