Math Is Fun Forum

Mirjam

Guest

Question on outcome.

I need a little help.

In the question below (-5)2 becomes 25 and 7 x (-5) becomes -35.
How come (-5)2 becomes 25; a positive outcome? Shouldn't the outcome be -25?

Thanks in advance.

Put "(-5)" where "y" is:

3 × (-5)2 + 7 × (-5) - 4
= 3 × 25 + 7 × (-5) - 4
= 75 - 35 - 4
= 36

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Question on outcome.

Hi Mirjam;

Welcome to the forum!

That answer is not correct.

(-5)2 does not equal 25.

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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.

gAr

Member

Registered: 2011-01-09

Posts: 3,482

Re: Question on outcome.

He probably wanted to say (-5)^2

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"Believe nothing, no matter where you read it, or who said it, no matter if I have said it, unless it agrees with your own reason and your own common sense"  - Buddha?

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bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Question on outcome.

Mirjam:

Math is a language, the most precise language there is. You must be exact in how you write it.

Supposing I did not know that the expression equals 36, then I am definitely going to say (-5)2 = - 10.

Supposing you had input that into a computer. There again without a doubt it would either be a syntax error or interpreted as (-5)*2 = - 10.

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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.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,583

Re: Question on outcome.

hi Mirjam

It looks like you are trying to evaluate this quadratic

with y = -5

when you do - 5 x -5 the answer is plus, because a minus times a minus makes a plus.

so

It's all because a minus times a minus makes a plus.

Bob

Last edited by Bob (2011-07-28 20:45:16)

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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

MathsIsFun

Administrator

Registered: 2005-01-21

Posts: 7,713

Re: Question on outcome.

See: Multiplying Negatives Makes A Positive

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"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman