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VCE Stuff => VCE Mathematics => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Mathematical Methods CAS => Topic started by: lacoste on June 14, 2009, 12:53:43 pm

Title: CAS calc help
Post by: lacoste on June 14, 2009, 12:53:43 pm: i can not solve this using the solve function and it comes out as false all the time.

the equation i want to solve is: (7x/root(x^2+25)) -16 = 0

to find the minimum pts for calculus.

any help appreciated!

thanks
Title: Re: CAS calc help
Post by: hyperblade01 on June 14, 2009, 01:28:41 pm: Is that equation the original or the equation of the derivative?
Title: Re: CAS calc help
Post by: lacoste on June 14, 2009, 06:18:03 pm: derivative, trying to find the min pt

i want to make the derivative equal to zero on the solve function and then get x
Title: Re: CAS calc help
Post by: hyperblade01 on June 14, 2009, 06:39:46 pm: $\frac{7x}{\sqrt {x^2+25}} - 16=0$

$7x = 16\sqrt{x^2+25}$

$\frac {7x}{16} = \sqrt{x^2+25}$

$\frac{49x^2}{256}= x^2 + 25$

$0 = \frac{207x^2}{256}+25$

$x^2 = \frac{-6400}{207}$

Not possible as you can't square root a negative, there's no stationary point? So then there isn't a minimum
Title: Re: CAS calc help
Post by: Over9000 on June 14, 2009, 06:56:51 pm: wrong statement, you can not square root a negative over R.
Title: Re: CAS calc help
Post by: GerrySly on June 14, 2009, 07:03:26 pm: Quote from: Over9000 on June 14, 2009, 06:56:51 pm
wrong statement, you can not square root a negative over R.
You can't square root a negative number in maths methods :P
Title: Re: CAS calc help
Post by: Mao on June 15, 2009, 09:59:35 am: Quote from: Over9000 on June 14, 2009, 06:56:51 pm
wrong statement, you can not square root a negative over R.

or N, Z, Q and various other sets :)