Determine rational functions from conditions 1
The graph of a rational function \(g\) is shown. It has the form
\(g(x)=2+\frac{k}{x+1}\), where \(k\ne0\).
a) Use the graph to read the x- and y-intercepts.
b) Use the y-intercept from part a) to determine \(k\). Then verify the x-intercept algebraically.
c) Explain why \(x=-1\) cannot be an x-intercept.
Hints
- Read the coordinates where the curve crosses each coordinate axis.
- At the y-intercept, the input is \(0\); use that graph-read point in the given function form.
- After finding \(k\), rewrite the function as one fraction before checking the x-intercept.
- An x-intercept must be a point in the function's domain.
Solution
1. From the graph, the y-intercept is \((0,-6)\) and the x-intercept is \((3,0)\).
2. At the y-intercept, \(x=0\). Substitute \((0,-6)\) into the given form: \(-6=2+\frac{k}{1}\), so \(k=-8\).
3. Thus, \(g(x)=2-\frac{8}{x+1}=\frac{2x-6}{x+1}\).
4. For the x-intercept, set the numerator equal to zero: \(2x-6=0\), so \(x=3\). Since \(3+1\ne0\), the x-intercept is \((3,0)\), agreeing with the graph.
5. At \(x=-1\), the denominator is zero, so \(g\) is undefined there. Therefore, \(x=-1\) cannot be an x-intercept.
Answer
a) x-intercept: \((3,0)\); y-intercept: \((0,-6)\)
b) \(k=-8\); the exact x-intercept is \((3,0)\)
c) \(x=-1\) is excluded from the domain because the denominator is zero.
