Around a given operating point, you can approximate any function with a polynomial (http://en.wikipedia.org/wiki/Taylor_series http://en.wikipedia.org/wiki/Tayl

Around a given operating point, you can approximate any function with a polynomial (http://en.wikipedia.org/wiki/Taylor_series http://en.wikipedia.org/wiki/Taylor_series). This includes approximating a polynomial with a lower-order polynomial. A line is first-order polynomial. Therefore, around a given operating point, a line can approximate a function. The key phrase there is "around a given operating point". If you zoom in close enough, a line and an exponential will appear to be the same. Outside of this area, they will clearly diverge. So you can't really say that a curved line is equivalent to a straight line, but you can say that sin(x) = x, for small values of x (and tolerable levels of error in sin(x)). In fact, that comes up quite often in engineering.