Question

What is the asymptote of the following function: Y = e^x-x

Answer 1

Answer:

y = -x

Step-by-step explanation:

Breaking down the equation

$y=e^x-x$ can be written as $y=e^x + (-x)$, making it more clear that this function is composed of two functions:

• An exponential function, and
• a linear function

asymptotes of the parts

The regular exponential function normally has an asymptote at y=0, a horizontal line because as "x" gets more and more negative, e^(x) gets closer and closer to zero (see blue graph).  This asymptotic behavior perhaps can be more easily understood by recognizing that $e^{-x}$ is equivalent to $\dfrac{1}{e^x}$, so that as the x values get more and more negative, it is effectively dividing 1 by a larger and larger number, thus getting closer and closer to zero.

In this situation, that exponential function is added to "-x", which is a linear function with a slope of negative 1.

As the left end of the exponential function gets closer and closer to zero, the exponential part contributes almost nothing to the sum, whereas as the "x" gets more and more negative, the linear function "-x" gets more and more positive (linearly).  Since the exponential piece adds almost nothing to it (when moving to the left), the combined function is essentially "-x" (although it is ever so slightly greater than it, just as it was ever so slightly greater than zero) (see pink graph, with dashed asymptote).

Clarifications

To be clear, on the right side of the graph, the exponential part of the function completely overwhelms the linear piece, and the function behaves much more like an exponential function, nearly completely masking the behavior of the linear piece.  Since an exponential function is unbounded, and the domain of both the exponential and linear functions are all real numbers, on the right side of the graph, there is no asymptotic behavior.