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

Question

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.

Answer 1

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{5² - 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