Maya claims, “If the vertex of a translation of \(y=x^2\) lies on the x-axis, then the function expression can be written as the square of a binomial.”
Test the claim by analyzing each function. State whether its vertex lies on the x-axis and give the vertex coordinates.
a) \(f(x)=x^2-14x+49\)
b) \(g(x)=x^2+5x+6.25\)
c) \(h(x)=x^2-2x-1\)
Hints
- Look for perfect-square trinomials.
- What does vertex form look like when the vertex is on the x-axis?
- What y-coordinate does every point on the x-axis have?
- Complete the square when an expression is not already a perfect square.
Solution
1. For \(f(x)=x^2-14x+49\), factor the perfect-square trinomial: \(f(x)=(x-7)^2\). The vertex is \((7, 0)\), so it lies on the x-axis.
2. For \(g(x)=x^2+5x+6.25\), factor the perfect-square trinomial: \(g(x)=(x+2.5)^2\). The vertex is \((-2.5, 0)\), so it lies on the x-axis.
3. For \(h(x)=x^2-2x-1\), complete the square: \(h(x)=x^2-2x+1-2=(x-1)^2-2\). The vertex is \((1, -2)\), so it does not lie on the x-axis.
4. In general, a translation of \(y=x^2\) with vertex \((r, 0)\) has equation \(y=(x-r)^2\). Therefore, Maya's claim is correct.
Answer
Maya's claim is correct.
a) Yes; the vertex is \((7, 0)\).
b) Yes; the vertex is \((-2.5, 0)\).
c) No; the vertex is \((1, -2)\).