Question

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

Answer 1

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.