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I need to check my answer for practice problems for up coming final

Select the approximate values of x that are solutions to f(x)=0, where f(x)=-9x^2+5x+7

A. (1.20,-0.65)

B. (-9,5)

C. (-0.56,-0.78)

D. (-1.29,0.71)
 
I believe the answer is D. This is not my final I have two weeks to cram for the test and need to refresh on topics earlier in the course. My math Partner came up with a different answer and we do not know who is correct or if we skipped a step.
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1 Answer

You should use the quadratic formula to solve this problem. 
 
If f(x) = ax2 + bx + c = 0 then x = [-b +- sqrt(b2 - 4ac)]/2a
 
So we have -9x^2+5x+7 = 0. We just substitute in to our equation 
 
x = [-5 + sqrt{5- 4(-9)(7)}/[2(-9)] and x = [-5 - sqrt{52 - 4(-9)(7)}/[2(-9)]
x = [-5 + sqrt{25 +252]/-18 and x = [-5 - sqrt{25 +252]/-18
x = [-5 + sqrt{277}]/-18 and x = [-5 - sqrt{277}]/-18
x ~ . 65 and x ~ 1.20
 
A

Comments

Krystal thank you so much. I realized I skipped some steps, which caused me to get the wrong answer. I really appreciate the way you showed step by step what to do. This has been a great learning experience.

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