Math Is Fun Forum

BarandaMan

Guest

Small question on inequalities!

Hey!
Hope you all are doing good.

I have

(6/5) > (2/5) - (2/5)z

I get this down to z > 2.

The answer says it should be, z < 2.

Please help me? Thank you!!!!

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Small question on inequalities!

Hi BarandaMan;

I am getting z > -2.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

BarandaMan

Guest

Re: Small question on inequalities!

Really?!

Can you please show me your working?

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: Small question on inequalities!

hi BarandaMan

x by 5

Add 2z

take 6

divide by 2

I suspect you ran into difficulty because of this

take 2

divide by -2

when you multiply or divide by a negative amount you must reverse the inequality sign

Bob

If you want to explore this rule some more, post back.

hi bobbym, sorry didn't know you were posting too.

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

BarandaMan

Guest

Re: Small question on inequalities!

Thank you very very much indeed. The change of the inequality sign is what got me. Thank you for helping me.

This makes sense. With this rule though, I just must remember it, right? Does it exist for a reason? Or is it just one we must remember?

Thanks!!!!

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: Small question on inequalities!

Definitely a reason.  Maths is like that !!!!!

Explanation coming up

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Bob

Administrator

Registered: 2010-06-20

Posts: 10,646

Re: Small question on inequalities!

OK.

Take a look at the diagram

On the right I am trying to show that 3 < 9

When you multiply or divide by -1 it becomes   -9 < -3

Make up your own example.  When you multiply or divide by a negative it has the effect of reflecting in the 'y axis' so the inequality flips around.

Hope that makes sense.

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

BarandaMan

Guest

Re: Small question on inequalities!

Wow, this is incredible. Thank you really. I don't know why people don't get taught stuff like this sometimes, or maybe I forgot, but never the underlying reasons for the rules it seems. This actually makes sense. Thank you. I don't think I will forget it again. I drew it out with three examples!!