Algebra

posted by Charly

I was given this answers to a problems I solved but I do not understand what she means hope you can help.


Before attempting to solve this quadratic equation, determine how many solutions there will be for this quadratic equation. Explain your reasoning. Finally, solve the equation.
(x - 9)2 = 81

I too, came up with the same answer and agree that this is a positive number and we should expect there to be two solutions to this problem. Carrying out the square is exactly the same as using the quadratic formula it is where you substitute a, b and c into the formula. It gives you the same answer to the polynomial used. This is because the quadratic formula came from finishing the square. Completing the square method is like the father of the quadratic formula.

(x - 9)2 = 81
(x - 9) = ã81
x = 9}9
x = 18 or x = 0

This answer was inreponce to another student who came up with the same as i did.


Hi Charly,

Yes, your answer is correct. Nice work. However, the method you used is not completing the square. Which one might it be, do you think?

  1. PsyDAG

    For you information, online "^" indicates an exponent, e.g., x^2 = x squared.

    (x - 9)^2 = 81

    x^2 -18x + 81 = 81

    x^2 - 18x = 0

    x(x-18) = 0

    X = 18 or 0

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