Core Connections Algebra 1, 2013
CC
Core Connections Algebra 1, 2013 View details
3. Section 11.3
Continue to next subchapter

Exercise 120 Page 564

To find the x-intercepts you will need to substitute 0 for y and solve for x.

Practice makes perfect

We want to graph the parabola by finding its x-intercepts and vertex. To do this, we will now follow four steps.

  1. Find and plot the x-intercepts.
  2. Find and graph the axis of symmetry.
  3. Find and plot the vertex.
  4. Draw the parabola through the vertex and the points where the x-intercepts occur.

Let's go through these steps one at a time.

Find and Plot the x-intercepts

To find the x-intercepts, think of the points where the graph of an equation crosses the x-axis. The y-value of that ( x, y) coordinate pairs is 0, and the x-values are the x-intercepts. To find the x-intercepts of the equation we should substitute 0 for y and solve for x. 0=3x^2-10x^2+2To solve the above quadratic equation for x we will use the Quadratic Formula. 0= ax^2+ bx+ c ⇕ x=- b± sqrt(b^2-4 a c)/2 a We first need to identify the values of a, b, and c. 0=3x^2-10x^2+2 ⇕ 0= 3x^2+( - 10)x+ 2 We see that a= 3, b= - 10, and c= 2. Let's substitute these values into the Quadratic Formula.
x=- b±sqrt(b^2-4ac)/2a
x=- ( -10)±sqrt(( - 10)^2-4( 3)( 2))/2( 3)
Solve for x and Simplify
x=10±sqrt((- 10)^2-4(3)(2))/2(3)
x=10±sqrt(100-4(3)(2))/2(3)
x=10±sqrt(100-24)/6
x=10±sqrt(76)/6
x=10±sqrt(4* 19)/6
x=10± sqrt(4)* sqrt(19)/6
x=10± 2 sqrt(19)/6
x=2(5± sqrt(19))/6
x=5± sqrt(19)/3
Using the Quadratic Formula, we found that the solutions of the equation are x_1= 5-sqrt(19)3 ≈ 0.21 and x_2= 5+sqrt(19)3 ≈ 3.12. Therefore, the x-intercepts of the parabola occur approximately at ( 0.21,0) and ( 3.12,0).

Find and Graph the Axis of Symmetry

The axis of symmetry is halfway between (x_1,0) and (x_2,0). Since we know that x_1 = 5-sqrt(19)3 and x_2 = 5+sqrt(19)3, the axis of symmetry of our parabola is halfway between ( 5-sqrt(19)3,0) and ( 5+sqrt(19)3,0). x=x_1+x_2/2 ⇓ x=5-sqrt(19)3+ 5+sqrt(19)3/2=103/2=5/3 We found that the axis of symmetry is the vertical line x= 53.

Find and Plot the Vertex

Since the vertex lies on the axis of symmetry, its x-coordinate is 53. To find the y-coordinate, we will substitute 53 for x in the given equation.
y=3x^2-10x+2
y=3( 5/3)^2-10( 5/3)+2
Simplify right-hand side
y=3(25/9)-10(5/3)+2
y=75/9-50/3+2
y=25/3-50/3+2
y=- 25/3 +2
y=- 25/3 +6/3
y=- 19/3
The y-coordinate of the vertex is - 193. Therefore, the vertex is the point ( 53,- 193).

Draw the Parabola

Finally, we will draw the parabola through the vertex and the x-intercepts.