In order to find the vertex of a quadratic function, a graph must already be displayed on the screen. To learn how to enter and graph a function, go to Graphing a Function. Consider the quadratic function, , which is graphed below in a standard window:
For this function, the vertex is the minimum point. To find the vertex, press 2nd TRACE [CALC], then select 3:Minimum by pressing 3.
The above last screen is asking for a left bound. Use the Left Arrow Key to move the flashing cursor slightly to the left of the minimum point and then press ENTER. The left bound is now marked, as shown below, with a small arrow towards the top of the screen.
The above last screen is asking for a right bound. Now use the Right Arrow Key to move the flashing cursor slightly to the right of the minimum point and then press ENTER.
Note that both the left bound and right bound are marked with a small arrows towards the top of the screen as is seen above. That screen asks for a guess.
Move the curser as close to the minimum point as possible, and press ENTER again. The last screen shows that the vertex is about (0.5, - 6.25).
To find the vertex of a parabola that opens downward instead of upward, repeat the above process, except select 4:Maximum instead of 3:Minimum.