Math Is Fun Forum

Maou-Sama

Member

Registered: 2015-03-03

Posts: 9

inequilities

Is the answer -1 ≤x ≤5
or
x ≤-1 , x≥5

if its the latter why isnt it the former one

Last edited by Maou-Sama (2015-03-08 03:57:48)

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: inequilities

hi Maou-Sama

Start by putting y = (x+1)(5-x) and thinking about the graph.

(1) It's a quadratic so it will be a parabola.

The x^2 term is negative, so for very large (+ or -) x, y will be negative.

So the shape is 'n' shaped rather than 'u' shaped.

It goes through x = -1 and + 5 on the x axis.

That's enough to sketch the graph as your book has done.

When is y negative.  Well on the left y is negative until x = -1; after that y is positive until x = 5.  After that, negative again.

So x  ≤ -1 and separately x ≥ +5.

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

Olinguito

Member

Registered: 2014-08-12

Posts: 649

Re: inequilities

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(multiplying both sides by

and reversing the inequality)

So the answer is

. The book is correct.

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Bassaricyon neblina

Maou-Sama

Member

Registered: 2015-03-03

Posts: 9

Re: inequilities

thanks a lot as always bob

Maou-Sama

Member

Registered: 2015-03-03

Posts: 9

Re: inequilities

thanks olinguito