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 January 27th, 2011, 09:05 AM #1 Newbie   Joined: Jan 2011 Posts: 4 Thanks: 0 Radicals I just wanted to know the answer to a specific question: sqrt15 sqrt21 / sqrt5 The answer I got was 9sqrt7 Any help is greatly appreciated, thank you. jrobbo
 January 27th, 2011, 09:13 AM #2 Senior Member     Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,155 Thanks: 462 Math Focus: Calculus/ODEs Re: Radicals You are given: $\frac{\sqrt{15}\sqrt{21}}{\sqrt{5}}$ Since $\sqrt{a}\sqrt{b}=\sqrt{ab}$ we may write this as: $\frac{\sqrt{3^2\cdot5\cdot7}}{\sqrt{5}}=\frac{\sqr t{3^2}\sqrt{5}\sqrt{7}}{\sqrt{5}}=3\sqrt{7}$
 January 27th, 2011, 09:14 AM #3 Global Moderator     Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4 Re: Radicals Since ?(15) = ?(5)*?(3), we can cancel the ?(5) from top and bottom and be left with ?(3)*?(21) = ?(3)*?(3)*?(7) = 3?(7) And Mark strikes again!
January 27th, 2011, 09:18 AM   #4
Senior Member

Joined: Jul 2010
From: St. Augustine, FL., U.S.A.'s oldest city

Posts: 12,155
Thanks: 462

Math Focus: Calculus/ODEs

Quote:
 Originally Posted by The Chaz ... And Mark strikes again!

It never hurts to have more than one way to look at a given problem.

 January 27th, 2011, 11:28 AM #5 Global Moderator     Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4 Re: Radicals ... or more than one way to use the same "formula"!
 January 28th, 2011, 09:30 AM #6 Newbie   Joined: Jan 2011 Posts: 4 Thanks: 0 Re: Radicals thanks guys
 January 28th, 2011, 10:11 AM #7 Global Moderator     Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4 Re: Radicals You are welcome.
 February 10th, 2011, 04:13 AM #8 Newbie   Joined: Jan 2011 From: Houston, Texas Posts: 13 Thanks: 0 Re: Radicals I did the same thing via video on my blog and youtube, but I won't include that bit. Here's a couple of other videos about radicals and why (and when) you can use them to do what we did here in this thread. I put these before and after my solution, so they are directly related, btw. Rednecks falling in love explanation: http://www.youtube.com/user/idearella#p/u/2/jUqYlK7mPgg Having stupid addition in the mix: http://www.youtube.com/user/idearella#p/u/0/Uawf6pgGG7o

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