The functions \(f(x)=\frac{1}{2}x+1\) and \(g(x)=\frac{1}{2}x-2\) are given.
a) Explain why the graphs of \(f\) and \(g\) are parallel.
b) A third line is \(h(x)=-x+b\). It must intersect \(f\) at the point on \(f\) whose x-coordinate is \(2\). Find \(b\).
c) The graphs of \(f\), \(h\), \(g\), and the y-axis bound a quadrilateral. Find its area.
Hints
- Compare the slopes and y-intercepts of \(f\) and \(g\).
- Use the stated x-coordinate to locate the required intersection point of \(f\) and \(h\).
- Find every vertex of the bounded quadrilateral before computing its area.
- Choose a coordinate-area method that works for a non-axis-aligned quadrilateral.
Solution
1. Lines \(f\) and \(g\) both have slope \(\frac{1}{2}\) and have different y-intercepts, so they are distinct parallel lines.
2. At \(x=2\), \(f(2)=\frac{1}{2}\cdot2+1=2\). Since \(h\) passes through \((2,2)\), \(2=-2+b\), so \(b=4\) and \(h(x)=-x+4\).
3. The y-axis meets \(f\) at \((0,1)\) and \(g\) at \((0,-2)\). Line \(h\) meets \(f\) at \((2,2)\).
4. To find the remaining vertex, solve \(-x+4=\frac{1}{2}x-2\). This gives \(x=4\) and \(y=0\), so the fourth vertex is \((4,0)\).
5. Using the vertices in order \((0,1)\), \((2,2)\), \((4,0)\), \((0,-2)\), the shoelace formula gives area \(\frac{1}{2}|-8-10|=9\) square units.
Answer
a) They are parallel because both have slope \(\frac{1}{2}\) and different y-intercepts.
b) \(b=4\)
c) \(9\) square units