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I need to check my answer for this practice problem.

Select the approximate values of x that are solutions to f(x) = 0, where f(x)=-8x^2 +9x+5.

A. (-1.60,1.80)

B. (-8,9)

C. (-1.13,-0.63)

D. (-0.41,1.53)
I believe the answer is A. My math partner thinks it is D. Who is right and did we miss a step?


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2 Answers

You and Christine need to actually DO problems yourself using the ideas we give you. That's the only way you will LEARN how to do them so that you can do similar problems yourself.
And you are NOT going to be prepared for a test if all you do is keep asking us to solve problems for you.
1. Use algebra to get the (function's corresponding) equation into this form: (a)x^2 + (b)x + (c) = 0.
2. Identify the values of a, b, and c in the equation; they are the coefficients of x^2 and x in the case of a and b, respectively, and c is the constant term. Note that any negative signs must be part of these constants leaving only positive signs to the left of them.
3. Substitute the values of a, b, and c into the formula:
x = (-(b)±sqrt((b)^2-4(a)(c)))/(2(a)). KEEP ALL the parentheses; just substitute.
4. NOW simplify your result using arithmetic and algebra.
5. Check your answers by substituting each one (there may be 1 or 2) back into the original function / equation. If you get zero then you have a correct answer. You don't need an answer sheet.
-8 X^2 +9X +5 =0
 X = -9/-16 ± √( 81+ 160)/16   (1)
   This is use of quadratic formula:
     In equation :
    Y=  aX^2 + bX +c
      Roots are :
      X = -b/2a ±√( b- 4ac) /2a
           Using formula will give the roots values of
            ( - 0.41, 1.53), calculate by calculator of (1)
         So the correct answer is D.


I am even more confused than before, someone please help. Am I right or could you show me how to do this properly.