
Concept explainers
To find : two quadratic function with the given x-intercepts

Answer to Problem 69E
The quadratic function opening upwards is f(x)=2x2+7x+3
The quadratic function opening downwards is g(x)=−2x2−7x−3
Explanation of Solution
Given information : (−3, 0), (−1/2,0)
Concept Involved:
The quadratic function will be of the form f(x)=ax2+bx+c where a, b and c are real numbers with a≠0 . The x-intercepts are the points where the graph of the function cuts the x-axis.
The quadratic function can also be written in the form f(x)=a(x−m)(x−n) , where m and n are the x -intercepts. If a<0 {negative} , then the graph of function opens downwards. If a>0 {positive} , then the graph of function opens upwards.
Calculation:
To find a quadratic function that opens UPWARDS with x-intercepts (−3, 0), (−1/2,0) , substitute a=2{positive} , m=−3 & n=−1/2 in f(x)=a(x−m)(x−n)
f(x)=2(x−(−3))(x−(−1/2))
Expand and rewrite the function
f(x)=2(x+3)(x+12)
f(x)=(x+3)(2x+1)
f(x)=2x2+x+6x+3
f(x)=2x2+7x+3
To find a quadratic function that opens DOWNWARDS with x-intercepts (−3, 0), (−1/2,0) , substitute a=−2{negative} , m=−3 & n=−1/2 in g(x)=a(x−m)(x−n)
g(x)=−2(x−(−3))(x−(−1/2))
Expand and rewrite the function
g(x)=−2(x+3)(x+12)
g(x)=−1(x+3)(2x+1)
g(x)=−1(2x2+x+6x+3)
g(x)=−1(2x2+7x+3)
g(x)=−2x2−7x−3
Graph:
Interpretation:
The function f(x)=2x2+7x+3 cuts the x-axis at x=−3, −12 and opens upward
The function g(x)=−2x2−7x−3 cuts the x-axis at x=−3, −12 and open downwards
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