# Math Help - Ratios/equations

1. ## Ratios/equations

If a:b=3:5, find the value of $\dfrac{a}{\sqrt{b^2-a^2}}$

2. Originally Posted by requal
If a:b=3:5, find the value of $\dfrac{a}{\sqrt{b^2-a^2}}$
note that we have $\frac ab = \frac 35$, that is, $a = \frac 35b$

replace $a$ with this expression in $\frac a{\sqrt{b^2 - a^2}}$ and simplify

3. i dont think that works as (3/5b)^2 is larger than b^2 thus creating a negative number under a square root?

4. Originally Posted by requal
i dont think that works as (3/5b)^2 is larger than b^2 thus creating a negative number under a square root?
1 - 9/25 is positive. so you have 1b^2 - (9/25)b^2 = (16/25)b^2. how is this negative?

5. yep, sorry I had mental diaharea thanks anyway