# A few pre calc questions! help appreciated

• Dec 13th 2008, 08:28 PM
Murphie
A few pre calc questions! help appreciated
f(x) = 3x^2-5 and g(x) = 2/x. find the function of f(g(x)) and give the domain.

Solve equation with exact answers: -x^2-3x+10 = 0

f(x) = ln(4-x) find the domain

Help appreciated
Thanks!
• Dec 13th 2008, 09:49 PM
earboth
Quote:

Originally Posted by Murphie
f(x) = 3x^2-5 and g(x) = 2/x. find the function of f(g(x)) and give the domain.

Solve equation with exact answers: -x^2-3x+10 = 0

f(x) = ln(4-x) find the domain

Help appreciated
Thanks!

1) Substitute the x in f by the term of g:

$f(x)=3\cdot \left(\dfrac2x\right)^2-5 = \dfrac{12}{x^2}-5$

Since the division by zero is not allowed the domain is $D=\mathbb{R}\setminus \{0\}$

2) $-x^2-3x+10 = 0$

Use the quadratic formula to solve this equation. I've got $x = -5~\vee~x = 2$

3) You are supposed to know that the ln-function is defined for positive arguments only. Thus the domain is $D = \{ x | 4-x > 0\}_{ \mathbb{R}}$