McGraw Hill Integrated II, 2012
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McGraw Hill Integrated II, 2012 View details
1. Graphing Quadratic Functions
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Exercise 73 Page 102

What happens with the vertex of a parabola when the parabola opens downward?

Counterexample:

Practice makes perfect

We will provide a counterexample to show that the following statement is false.

The vertex of a parabola is always the minimum of the graph.

Let's recall that a graph of a quadratic function, y=ax^2+bx+c, opens upward whenever a is positive, and opens downward whenever a is negative.

In the case that a>0, the vertex of the parabola is its minimum. In the case that a<0, the vertex of the parabola is its maximum.

This tells us that any parabola that opens downward is a counterexample for the given statement.