Question

What belongs in the box marked with the question mark in the proof?Given: 3x - 4 = 2(x + 8), Prove: x = 20

Answer 1

http://prntscr.com/cojcl here is the image.

Answer 2

@SmoothMath can you help?

Answer 3

Would it be The Distributive Property?

Answer 4

adding 4 on both sides

Answer 5

This is a Two-Column proof, the reasons have to be postulate, theorems, properties, etc.

Answer 6

@zepp I've heard you're good with two column proofs

Answer 7

We've send 2x to the left side, I believe it should be Associative Property of Addition

3x = 2x + 20
3x + 0 = 2x + 20

Answer 8

Zepp, the question mark is in the column after the statement 3x = 2x + 20
so it needs to be a reason for going from
3x-4 = 2x + 20 to
3x = 2x+20

You should ask yourself how it is that we're able to get from one to the other, and the answer, as rabhajishek point out, is that we added 4 to both sides. So that's what we DID, but what is it that ALLOWS us to do that?

The associative property of addition isn't it, I wouldn't say. That's the one that would allow us to rewrite
(a+b)+c as
a+(b+c)

Instead, there's a property that lets us add something to both sides of an equation without unbalancing it. What's that property called?
The addition property of equality.

Answer 9

Sorry for the long response time, Feara. I was nappin' :3

Answer 10

Oh I see :D

Answer 11

Thanks for the clarification @SmoothMath