# Ratios/equations

• Jan 22nd 2009, 09:37 PM
requal
Ratios/equations
If a:b=3:5, find the value of $\dfrac{a}{\sqrt{b^2-a^2}}$
• Jan 22nd 2009, 10:35 PM
Jhevon
Quote:

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
• Jan 23rd 2009, 12:26 AM
requal
i dont think that works as (3/5b)^2 is larger than b^2 thus creating a negative number under a square root?(Wondering)
• Jan 23rd 2009, 01:13 AM
Jhevon
Quote:

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?(Wondering)

1 - 9/25 is positive. so you have 1b^2 - (9/25)b^2 = (16/25)b^2. how is this negative?
• Jan 23rd 2009, 03:36 AM
requal
yep, sorry I had mental diaharea(Crying) thanks anyway