SOLUTION: 1. If y = a(x - 3)^2 + c and y = (2x - 3)(2x + b) represent the same quadratic function, find a, b, and c. (There are two possibles methods: i) expand both or ii) use the symmetry

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equation Customizable Word Problems -> SOLUTION: 1. If y = a(x - 3)^2 + c and y = (2x - 3)(2x + b) represent the same quadratic function, find a, b, and c. (There are two possibles methods: i) expand both or ii) use the symmetry       Log On


   



Question 79946: 1. If y = a(x - 3)^2 + c and y = (2x - 3)(2x + b) represent the same quadratic function, find a, b, and c. (There are two possibles methods: i) expand both or ii) use the symmetry of the roots around the vertex.)
- I expanded both equations but I could not simplify the terms.
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Hello there,
Thank you in advance for your help. For the following problem, I was able to solve it using method 2, but I do not know how to solve it using method 1.
Best,
James

Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
y%5B1%5D+=+a%28x-3%29%5E2%2Bc
y%5B2%5D+=+%282x-3%29%282x%2Bb%29
Expand both equations:
y%5B1%5D+=+a%28x%5E2-6x%2B9%29%2Bc
y%5B1%5D+=+ax%5E2-6ax%2B%289a%2Bc%29
y%5B2%5D+=+4x%5E2%2B%282b-6%29x-3b
Since these two equations represent the same quadratic function, the corresponding terms should be equal, so comparing term-for-term,...
First term:
ax%5E2+=+4x%5E2
a+=+4
Second term:
-6ax+=+%282b-6%29x
-6a+=+2b-6 Substitute a = 4.
-6%284%29+=+2b-6
-24+=+2b-6
2b+=+-18
b+=+-9
Third term:
9a%2Bc+=+-3b Substitute a = 4 and b = -9
9%284%29%2Bc+=+-3%28-9%29
36%2Bc+=+27
c+=+-9
Check: Substitute a = 4, b = -9, and c = -9
First equation:
y%5B1%5D+=+4%28x-3%29%5E2%2B%28-9%29
y%5B1%5D+=+4%28x%5E2-6x%2B9%29%2B%28-9%29
y%5B1%5D+=+4x%5E2-24x%2B%2836%2B%28-9%29%29
y%5B1%5D+=+4x%5E2-24x%2B27
Second equation:
y%5B2%5D+=+%282x-3%29%282x%2B%28-9%29%29
y%5B2%5D+=+%282x-3%29%282x-9%29
y%5B2%5D+=+4x%5E2-18x-6x%2B27
y%5B2%5D+=+4x%5E2-24x%2B27
As you can see, the two equations are identical if:
a = 4, b = -9, and c = -9