# Inequalities help

• March 26th 2013, 07:33 PM
calmo11
Inequalities help
I need help with these two inequalities Attachment 27702

For c) I can get to x>or=5 but I know that it is true when x<0 too but I don't know how to show that algebraically

For d) I can get to x>3/2 but I also know that it is true when x<1 but cant show it please help
• March 26th 2013, 08:44 PM
Soroban
Re: Inequalities help
Hello, calmo11!

Quote:

$(c)\;\;e^{x^2} \:\ge\:e^{5x}$

We have: . $x^2 \:\ge\:5x \quad\Rightarrow\quad x^2 - 5x \:\ge\:0$

. . . . . . . . $x(x-5)\:\ge\:0$

It says: the product of two number is positive.
This is true if both factors are positive or both factors are negative.
We must consider these two cases.

Both positive:
. . $x \,\ge\, 0 \:\text{ and }\:x-5\,\ge\,0 \quad\Rightarrow\quad x \,\ge\,5$
. . Both are true if $x\,\ge\,5$

Both negative:
. . $x\,\le\,0\:\text{ and }\:x-5 \,\le\,0 \quad\Rightarrow\quad x\,\le\,5$
. . Both are true if $x\,\le\,0$

Answer: . $(x\,\ge\,5)\,\text{ or }\,(x\,\le\,0)$

Quote:

$(d)\;\;|4x-5| \:>\:1$

The absolute inequality gives us two statements:

. . $[1]\;4x-5 \,>\,1\;\text{ or }\;[2]\;4x-5\,<\,-1$

Solve them separately:

. . $[1]\;4x - 5 \,>\,1 \quad\Rightarrow\quad 4x\,>\.6 \quad\Rightarrow\quad x \,>\,\tfrac{3}{2}$

. . $[2]\;4x-5 \,<\,-1 \quad\Rightarrow\quad 4x \,<\,4 \quad\Rightarrow\quad x \,<\,1$

Answer: . $(x\,<\,1)\:\text{ or }\:(x\,>\,\tfrac{3}{2})$
• March 26th 2013, 09:30 PM
calmo11
Re: Inequalities help
You are a genius! Why can't you teach me my uni course!