Question

Lin solved the equation 8 (x - 3) +7= 2x (4 - 17) incorrectly.

Find the 2 errors in her solution.
What should her answer have been?

Answer 1

x=1/2

Explanation:

8(x-3)+7=2x(4-17)

8(x-3)+7=2x(-13)

This is where Lin's first mistake appears, 4-17 is -13, not positive 13.

Next, Lin distributes.

8x-24+7=26x

This is right with Lin's equation, since Lin got 13 instead of -13.

However, if we were doing it correctly from the start, it should be:

8x-24+7=-26x

Okay, after 8x-24+7=26x Lin simplifies again to:

8x-17=26x

She then tries to eliminate 8x.

What she does wrong here, is that she subtracts 8x from the left, and adds 8x to the right.

This makes it so that the equation is not equal anymore, because what you do to one side, you must do to the other.

This is Lin's second mistake.

What she should have gotten is

-17=18x.

Okay, now that we have found her mistakes, let's solve this correctly.

8(x-3)+7=2x(4-17)

8(x-3)+7=2x(-13)

Distribute.

8x-24+7=-26x

8x-17=-26x

Add 17 to both sides.

8x=-26x+17

Add 26x to both sides.

34x=17

x=1/2

Her answer should have been 1/2.

Answer 2

Her final answer should have been x = 1/2.

Explanation:

Her 2 errors were that she didn't distribute the 2x to both 4 and -17 and that she made the 13 positive instead of negative when she added 4 and -17 together.

Correct solution:

8(x-3)+7=2x(4-17)

8x-24+7=8x-34x

-17=-34x

The negatives on both sides cancel each other out

x = 1/2