Quadratic formula explained (article) | Khan Academy
https://www.khanacademy.org/math/algebra/x2f8bb11595b61c86:quadratic-functions-equations/x2f8bb11595b61c86:quadratic-formula-a1/a/quadratic-formula-explained-article
WEBWorked example. First we need to identify the values for a, b, and c (the coefficients). First step, make sure the equation is in the format from above, a x 2 + b x + c = 0 : x 2 + 4 x − 21 = 0. a. is the coefficient in front of x 2. , so here a = 1. (note that a. can’t equal 0. -- the x 2. is what makes it a quadratic).
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