Analyze the Quadratic Function: Domains of y = -2x² + 3x + 2

Question

Find the positive and negative domains of the following function:

y=2x2+3x+2 y=-2x^2+3x+2

Step-by-Step Solution

To solve the given problem, we will perform the following steps:

  • Calculate the roots of the quadratic equation y=2x2+3x+2 y = -2x^2 + 3x + 2 using the quadratic formula: x=b±b24ac2a x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} .
  • Given a=2 a = -2 , b=3 b = 3 , and c=2 c = 2 , use the formula:

x=3±324(2)(2)2(2)=3±9+164=3±254 x = \frac{-3 \pm \sqrt{3^2 - 4(-2)(2)}}{2(-2)} = \frac{-3 \pm \sqrt{9 + 16}}{-4} = \frac{-3 \pm \sqrt{25}}{-4}

  • The roots are x=3+54=12 x = \frac{-3 + 5}{-4} = -\frac{1}{2} and x=354=2 x = \frac{-3 - 5}{-4} = 2 .
  • The roots 12-\frac{1}{2} and 22 divide the x-axis into three intervals: (,12) (-\infty, -\frac{1}{2}) , (12,2) (-\frac{1}{2}, 2) , and (2,) (2, \infty) .
  • Test a point from each interval in the function to determine the sign of the function in those intervals:

Choose x=1 x = -1 for interval (,12)(- \infty, -\frac{1}{2}):
Substitute into the function: y=2(1)2+3(1)+2=23+2=3 y = -2(-1)^2 + 3(-1) + 2 = -2 - 3 + 2 = -3 (negative).

Choose x=0 x = 0 for interval (12,2)(- \frac{1}{2}, 2):
Substitute into the function: y=2(0)2+3(0)+2=2 y = -2(0)^2 + 3(0) + 2 = 2 (positive).

Choose x=3 x = 3 for interval (2,)(2, \infty):
Substitute into the function: y=2(3)2+3(3)+2=18+9+2=7 y = -2(3)^2 + 3(3) + 2 = -18 + 9 + 2 = -7 (negative).

Therefore, the positive domain where the function is positive is 12<x<2 -\frac{1}{2} < x < 2 , and the negative domains are x<12 x < -\frac{1}{2} or x>2 x > 2 .

The solution to the problem is:

x>0:12<x<2 x > 0 : -\frac{1}{2} < x < 2

x>2 x > 2 or x<0:x<12 x < 0 : x < -\frac{1}{2}

Answer

x > 0 : -\frac{1}{2} < x < 2

x > 2 or x < 0 : x < -\frac{1}{2}