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Lisa solved 3x2 + 1 = 28 as shown. 3x2 + 1 = 28 3x2 = 27 x2 = 9 She concluded that the solutions to this quadratic equation are x = -9 or x = 9. Using a complete sentence, explain what mistake she made

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altavistard

Important:  Please use " ^ " to indicate exponentiation:   3x^2 + 1 = 28.

The correct solution follows:  

Subtract 1 from both sides:  3x^2 + 1 = 28  => 3x^2 = 27  => x^2 = 9, and x = plus or minus 3.

This person's solution was OK until the last step, where she mistakenly gave the sqrt of 9 the values -9 and +9.  Again, the correct answers are -3 and +3.

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