Graphing Quadratics F(X) = Ax2 + Bx + C
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2 Graphing Quadratics f(x) = ax + bx + c -- October 23, 2009// Slightly revised April 13, 2010
You already know:
The graph is a parabola.
It opens up if a > 0 and down if a < 0.
The y-intercept is (0,c).
The graph is symmetric about a vertical line through the vertex.
You may or may not know this: b FACT: The vertex is at x= -, y = f ( x ) V2a V V
Example 1: y = 2x2 – 12x + 5.
Opens up.
Vertex at x = –(-12)/4 = 3; y = 2*32 –12*3 + 5 = 18 – 36 + 5 = -13; vertex (3, -13).
Intercept is (0, 5).
New idea: Symmetric point to the intercept is (6, 5) -- reflect (0,5) through the axis of symmetry, x = 3.
We have three points: (3, -13), (0, 5), and (6, 5), and we know the parabola opens up. Sketch the curve by plotting the three points and drawing the parabola through them.
Here’s what we do:
1. Note whether it opens up or down. b 2. Find the vertex x= -, y = f ( x ) . V2a V V 3. Plot the intercept, and plot the symmetric point.
4. Sketch.
Example 2. y = –x2 + 10x –14
1. Opens down.
2 2. The vertex is at xV = -(10)/2(-1) = 5, yV = –(5 ) + 10*5 + 14 = 25 – 14 = 11. So, the vertex is at (5, 11). 3. Intercept (0, -14), and symmetric point (10, -14) -- reflected across the line x = 5.
4. Three points, (5, 11), (0, -14), (10, -14). Opens down. Sketch.
Example 3. y = 3x2 + 5x + 4 (Numbers not nice, but we can handle them.)
1. Opens up.
2. Vertex x = -5/(2*3) = -5/6; y = 3(-5/6)2 + 5(-5/6) + 4 = 25/12 – 25/6 + 24/6 = 23/12, a little less than 2. Vertex (-5/6, 2– ).
3. Intercept (0, 4); symmetric point (-5/3, 4) , because 2*(-5/6) = -5/3.
4. Three points, opens up; sketch.
In some special cases, other ideas make it easier:
Example 4 (Factored; or factors easily). y = (x – 3)(x + 9).
Since this is factored, the vertex must be half-way between the zeroes: xV = (3 + -9)/2 = -3; yV = (-3 – 3)(-3 + 9) = -36; Vertex (-3, -36).
Two more points are the zeroes, (3, 0) and (-9, 0).
Three points, opens up, sketch.
Example 5 (Easy to CTS): y = x2 – 10x + 1
We complete the square, but now adding and subtracting the same number on the right-hand side: y = x2 – 10x + 25 –25 + 1 = (x – 5)2 – 25 + 1 = (x – 5)2 – 24. Vertex at (5, -24).
As done previously, the y-intercept and its symmetric partner are (0, 1), (10, 1).
Three points, so sketch. (No fuss, no muss, no bother . . . )
The book uses CTS as the primary method – see the book’s Examples 1, 2, 3 in Section 2.4. The book’s Example 4 is done using the formula, like Examples 1, 2, 3 on this handout.
T. Wallgren – Spring 08 /Spring 2010