Math Is Fun Forum

  Discussion about math, puzzles, games and fun.   Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ • π ƒ -¹ ² ³ °

You are not logged in.

#1 2015-03-08 03:56:22

Maou-Sama
Member
Registered: 2015-03-03
Posts: 9

inequilities

aSulLpf.gif
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)

Offline

#2 2015-03-08 04:22:22

Bob
Administrator
Registered: 2010-06-20
Posts: 10,053

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


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 smile

Offline

#3 2015-03-08 04:36:18

Olinguito
Member
Registered: 2014-08-12
Posts: 649

Re: inequilities

[list=*]
[*]

[/*]
[/list]

(multiplying both sides by
and reversing the inequality)

So the answer is
. The book is correct.


Bassaricyon neblina

Offline

#4 2015-03-08 05:00:14

Maou-Sama
Member
Registered: 2015-03-03
Posts: 9

Re: inequilities

thanks a lot as always bob

Offline

#5 2015-03-08 05:01:30

Maou-Sama
Member
Registered: 2015-03-03
Posts: 9

Re: inequilities

thanks olinguito

Offline

Board footer

Powered by FluxBB