math
posted by jess .
We are learning inequalities and I just did some homework but the teacher returned it saying I did some of them wrong.
1. 2x  10 < 2
2. 2x + 15 < 17
I didn't flip the inequality sign when I divided by (2).
But why do we always have to flip the sign when we multiply or divide by a negative number?

In google type:
inequalities and negative numbers
You will find couple sites.
mathforum is good.
1.
2 x  10 < 2
2 x < 2 + 10
2 x < 12 Divide both sides with 2
x > 12 /  2
x >  6
2.
2x + 15 < 17
2 x < 17  15
2 x < 2 Divide both sides with 2
x > 2 / 2
x > 1