Maths Olympiad Prep

Library / /1 of 21

Algebra Difficulty 3.8 AMC 10/12 Find the answer United States

The graph of y=ex+1+e−x−2y = e^{x+1} + e^{-x} - 2 has an axis of symmetry. What is the reflection of the point (−1,12)(-1, \frac{1}{2}) over this axis?

Pick one

Solution

Let f(x)=ex+1+e−x−2f(x) = e^{x+1} + e^{-x} - 2. Because f(x)f(x) approaches infinity as ∣x∣|x| increases without bound, the only possible axis of symmetry is a vertical line. If the axis of symmetry has equation x=cx = c, then f(x)=f(2c−x)f(x) = f(2c-x) for every real xx, which is equivalent to e⋅ex+e−x=e2c+1e−x+e−2cexe \cdot e^x + e^{-x} = e^{2c+1}e^{-x} + e^{-2c}e^x. Multiplying through by exe^x and simplifying gives (e−e−2c)e2x=e2c+1−1(e - e^{-2c})e^{2x} = e^{2c+1} - 1. Because this equation holds for all xx, it follows that e−e−2c=0e - e^{-2c} = 0 and e2c+1−1=0e^{2c+1} - 1 = 0. Thus c=−12c = -\frac{1}{2}. The reflection of (−1,12)(-1, \frac{1}{2}) with respect to the vertical line x=−12x = -\frac{1}{2} is (0,12)(0, \frac{1}{2}).

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.