Thread: Solve linear equation

Hi,
I found the following practice problem online. I tried to solve it by doing what I was taught at school to solve such an equation-moving the variable to one side and the constants to another and I ended up with this:
5(-3x - 2) - (x - 3) = -4(4x + 5) + 13
-15x -10 -x + 3 = -16x -20 + 13
-15x -x +16x = 10 -3 +13
0 = 20

Obviously wrong!

But actually solution does this:
5(-3x - 2) - (x - 3) = -4(4x + 5) + 13
-15x -10 -x + 3 = -16x -20 + 13
-16x -7 = -16x -7
Multiply both sides by 16x + 7 and get:
0=0

Why does the conventional method not yield results when that is how I solve all my linear equations? How do I know when to apply the second method?

Thanks.

I'm really confused by your method and I'm sure you are confusing yourself. Make it easier by taking away 1 thing at a time:

Expand brackets to get:



Get the x values to the left side.

I added 16x to both sides.

Now the rest should be simple.

You are doing this:



See how you just wrote "16x" you may as well add it straight away to get yourself not confused...

Originally Posted by math934

Hi,
I found the following practice problem online. I tried to solve it by doing what I was taught at school to solve such an equation-moving the variable to one side and the constants to another and I ended up with this:
5(-3x - 2) - (x - 3) = -4(4x + 5) + 13
-15x -10 -x + 3 = -16x -20 + 13
-15x -x +16x = 10 -3 +13

Here's your error. You added 10- 3 to both sides but the right side is -20+ 13. You dropped the -20.

0 = 20

You should have 0= 20- 20 which is the same as 0 = 0.

Obviously wrong!
But actually solution does this:
5(-3x - 2) - (x - 3) = -4(4x + 5) + 13
-15x -10 -x + 3 = -16x -20 + 13
-16x -7 = -16x -7
Multiply both sides by 16x + 7 and get:
0=0

Why does the conventional method not yield results when that is how I solve all my linear equations? How do I know when to apply the second method?

Thanks.

HallsofIvy,
I knew it,I knew it, I knew it. This is my problem. I always make the silliest mistake and end up messing up the entire problem

Thanks very much for your help.