Question

Is the answer to the problem below correct? If yes, why? If not, what mistake(s) were made in solving the

problem an what is the correct answer?
Step 1: 12 = -2(x - 4) - 8
Step 2: 12 = -2x - 8 - 8
Step 3: 12 = -2x - 16
Step 4: 28 = -2x
Step 5: 26 = x

Answer 1

9514 1404 393

Answer:

x = -6 (correct answer)

mistakes were made in Step 2 and Step 5.

Explanation:

The mistake is made in Step 2, application of the distributive property. This is often a source of error. Here, the error is in signs.

Step 1: 12 = -2(x -4) -8

Step 2: 12 = -2x +8 -8 . . . . . . the correct step 2

Step 3: 12 = -2x . . . . . . . . . . . like terms combined

Step 4: -6 = x . . . . . . . . . . . . . divide by -2

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Check

12 = -2(-6 -4) -8 = -2(-10) -8 = 20 -8 = 12 . . . . answer checks OK

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Comment on other mistake

In the original Step 5, -2 was divided on the right and added on the left. This is a violation of the properties of equality, so guarantees a wrong answer. The same operation must be performed on both sides of the equation.

An additional mistake was made in that the answer was not checked.