Because with a non-linear function, the slope is usually never the same. For example, look at www.desmos.com/calculator/pbexodgxe2 As you see, none of the slopes are consistent.
why we can't use slope for non-linear function?
Thus, the graph of a nonlinear function is not a line. Linear functions have a constant slope, so nonlinear functions have a slope that varies between points. Algebraically, linear functions are polynomials with highest exponent equal to 1 or of the form y = c where c is constant.
Nonlinear Function: Definition & Examples - Video & Lesson ...
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