# Algebraic expressions

• March 20th 2011, 08:22 AM
ikislav
Algebraic expressions
Hello everyone.

I got the following problem:

If a *b != 0 and a != b, then the following expression:

$(\frac{(a-b)^2}{ab}+3)*(\frac{a}{b}-\frac{b}{a}):\frac{a^3-b^3}{ab}$

equals:
a) $a^2+ab+b^2$
b) $a-b$
c) $a+b$
d) $\frac{1}{a}-\frac{1}{b}$
e) $\frac{1}{a}+\frac{1}{b}$

So, I started like this:

$(\frac{(a-b)^2}{ab}+3)*(\frac{a}{b}-\frac{b}{a}):\frac{a^3-b^3}{ab}$

$(\frac{a^2-2ab+b^2}{ab}+3)*(\frac{a^2}{ab}-\frac{b^2}{ab})*\frac{ab}{a^3-b^3}$

$(\frac{a^2-2ab+b^2}{ab}+3)*\frac{a^2-b^2}{a^3-b^3}$

and that's where I'm stuck :(
• March 20th 2011, 09:00 AM
skeeter
Quote:

Originally Posted by ikislav
Hello everyone.

I got the following problem:

If a *b != 0 and a != b, then the following expression:

$(\frac{(a-b)^2}{ab}+3)*(\frac{a}{b}-\frac{b}{a}):\frac{a^3-b^3}{ab}$

equals:
a) $a^2+ab+b^2$
b) $a-b$
c) $a+b$
d) $\frac{1}{a}-\frac{1}{b}$
e) $\frac{1}{a}+\frac{1}{b}$

So, I started like this:

$(\frac{(a-b)^2}{ab}+3)*(\frac{a}{b}-\frac{b}{a}):\frac{a^3-b^3}{ab}$

$(\frac{a^2-2ab+b^2}{ab}+3)*(\frac{a^2}{ab}-\frac{b^2}{ab})*\frac{ab}{a^3-b^3}$

$(\frac{a^2-2ab+b^2}{ab}+3)*\frac{a^2-b^2}{a^3-b^3}$

and that's where I'm stuck :(

I assume the following is the correct problem statement ...

If $a \cdot b \ne = 0$ and $a \ne b$, then the following expression:

$(\frac{(a-b)^2}{ab}+3) \cdot (\frac{a}{b}-\frac{b}{a}) \div \frac{a^3-b^3}{ab}$

equals:
a) $a^2+ab+b^2$
b) $a-b$
c) $a+b$
d) $\frac{1}{a}-\frac{1}{b}$
e) $\frac{1}{a}+\frac{1}{b}$

...

$\left[\frac{a^2 - 2ab + b^2}{ab}+3\right] \cdot \frac{a^2-b^2}{a^3-b^3}$

$\left[\frac{a^2 - 2ab + b^2}{ab}+\frac{3ab}{ab}\right] \cdot \frac{(a-b)(a+b)}{(a-b)(a^2+ab+b^2)}$

$\frac{a^2 +ab + b^2}{ab} \cdot \frac{(a-b)(a+b)}{(a-b)(a^2+ab+b^2)}$

can you finish?
• March 20th 2011, 09:03 AM
Plato
Quote:

Originally Posted by ikislav
If a *b != 0 and a != b, then the following expression: $(\frac{(a-b)^2}{ab}+3)*(\frac{a}{b}-\frac{b}{a}):\frac{a^3-b^3}{ab}$

Note two things: $\left( {\dfrac{{a^2 - 2ab + b^2 }}{{ab}} + 3} \right) = \left( {\dfrac{{a^2 + ab + b^2 }}{{ab}}} \right)$

$a^3-b^3=(a-b)(a^2+ab+b^2)~.$
• March 20th 2011, 09:21 AM
ikislav
Quote:

Originally Posted by skeeter
I assume the following is the correct problem statement ...

If $a \cdot b \ne = 0$ and $a \ne b$, then the following expression:

$(\frac{(a-b)^2}{ab}+3) \cdot (\frac{a}{b}-\frac{b}{a}) \div \frac{a^3-b^3}{ab}$

equals:
a) $a^2+ab+b^2$
b) $a-b$
c) $a+b$
d) $\frac{1}{a}-\frac{1}{b}$
e) $\frac{1}{a}+\frac{1}{b}$

...

$\left[\frac{a^2 - 2ab + b^2}{ab}+3\right] \cdot \frac{a^2-b^2}{a^3-b^3}$

$\left[\frac{a^2 - 2ab + b^2}{ab}+\frac{3ab}{ab}\right] \cdot \frac{(a-b)(a+b)}{(a-b)(a^2+ab+b^2)}$

$\frac{a^2 +ab + b^2}{ab} \cdot \frac{(a-b)(a+b)}{(a-b)(a^2+ab+b^2)}$

can you finish?

Ok, so than that's:

$\frac{a+b}{ab}$

Is that right? But it is not among the given choice of answers.
• March 20th 2011, 09:27 AM
skeeter
Quote:

Originally Posted by ikislav
Ok, so than that's:

$\frac{a+b}{ab}$

Is that right? But it is not among the given choice of answers.

yes, it is ...
• March 20th 2011, 09:39 AM
ikislav
Quote:

Originally Posted by skeeter
yes, it is ...

Ok, if I multiply with $ab$, I get $a^2b+ab^2$. I am at a loss here.
• March 20th 2011, 09:41 AM
skeeter
$\dfrac{a+b}{ab} = \dfrac{a}{ab} + \dfrac{b}{ab} =$ ... one more step
• March 20th 2011, 10:27 AM
ikislav
Quote:

Originally Posted by skeeter
$\dfrac{a+b}{ab} = \dfrac{a}{ab} + \dfrac{b}{ab} =$ ... one more step

I got it:

$\dfrac{a+b}{ab} = \dfrac{a}{ab} + \dfrac{b}{ab} = \dfrac{1}{b} + \dfrac{1}{a}$

Right answer is the one under "e" :)