If 1/x+x=5, what does 1/x^2+x^2 equal?Date: 08/16/2002 at 23:04:56 From: Shawn Ellis Subject: Problem on GMAT (algebra) Dr. Math: I recently took the GMAT and came across the following problem: If 1/x + x = 5, then what does 1/x^2 + x^2 equal? The answers were 21, 22, 23, 24, 25. This seemingly easy problem really created a blank for me. I guessed 25, but I'm still not sure how to work it out. Any help would be appreciated! Shawn Date: 08/17/2002 at 00:47:36 From: Doctor Paul Subject: Re: Problem on GMAT (algebra) 1/x = 5-x so 1 = 5*x - x^2 x^2 - 5*x + 1 = 0 solve via quadratic formula x = [5 +- sqrt(21)]/2 so either x = 5/2 + sqrt(21)/2 or x = 5/2 - sqrt(21)/2 Now we pick one of these solutions at random and see what happens. Let's work with the solution x = 5/2 + sqrt(21)/2 Now we compute 1/x^2 + x^2. First of all, x^2 = 25/4 + 5*sqrt(21)/2 + 21/4 = 46/4 + 5*sqrt(21)/2 = [23 + 5*sqrt(21)]/2 Thus 1/x^2 + x^2 = 2/[23 + 5*sqrt(21)] + [23 + 5*sqrt(21)]/2 Obtain a common denominator: 4/(2*[23 + 5*sqrt(21)]) + [23 + 5*sqrt(21)]^2/(2*[23 + 5*sqrt(21)]) = 4 + [23 + 5*sqrt(21)]^2 ----------------------- = 2*[23 + 5*sqrt(21)] 4 + 529 + 230*sqrt(21) + 525 ---------------------------- = 46 + 10*sqrt(21) 1058 + 230*sqrt(21) ------------------- = 46 + 10*sqrt(21) 23*(46 + 10*sqrt(21) -------------------- = (46 + 10*sqrt(21) 23 If we had chosen instead to work with the solution x = 5/2 - sqrt (21)/2, the computations would have been different, but the answer comes out the same. Perhaps you can try it on your own. I hope this helps. Please write back if you'd like to talk about this some more. - Doctor Paul, The Math Forum http://mathforum.org/dr.math/ Date: 08/18/2002 at 22:18:58 From: Shawn Ellis Subject: Thank you (problem on GMAT (algebra)) Doctor Paul: Thank you so much! You make it look so easy. I now know I need to review the quadratic formula, especially when the problems aren't nice and tidy. In your opinion, do you think the average non-math major could do this problem in approximately 2 minutes? (That's what it would take to keep pace on the GMAT.) Thanks again! Shawn Tokyo Date: 08/18/2002 at 23:33:29 From: Doctor Paul Subject: Re: Thank you (problem on GMAT (algebra)) If your test allows the use of a scientific calculator, then I'd say this problem could be done in about 30 seconds. Once you solve for x, you can just plug in 1/x^2 + x^2 and you'll get 23. Without a calculator, I think it's not reasonable to be done in 2 minutes. But maybe I did it the hard way. If there's another way to do this problem, then 2 minutes might be reasonable. I hope this helps. Please write back if you'd like to talk about this some more. - Doctor Paul, The Math Forum http://mathforum.org/dr.math/ Date: 08/18/2002 at 23:51:46 From: Shawn Ellis Subject: Problem on GMAT (algebra) Doctor Paul: Thank you again for your prompt response! Calculators aren't allowed when taking the GMAT. So, I would imagine there is a simpler method to solve this problem, although I have NO idea what that might be... Shawn Tokyo Date: 08/19/2002 at 00:51:15 From: Doctor Greenie Subject: Re: Problem on GMAT (algebra) Hi, Shawn - I enjoyed reading the exchange between you and Dr. Paul on this problem. Yes, there is an easy way to do it, using a "trick" that is a favorite of mine. Given 1/x + x = 5 simply square both sides: (1/x + x)^2 = 5^2 1/x^2 + 2(1/x)(x) + x^2 = 25 1/x^2 + 2 + x^2 = 25 1/x^2 + x^2 = 23 Aggravatingly easy, isn't it...?! On the other hand, it is always exciting to find a simple way to tackle a type of problem which has had you stumped. - Doctor Greenie, The Math Forum http://mathforum.org/dr.math/ Date: 08/19/2002 at 01:59:59 From: Shawn Ellis Subject: Thank you (problem on GMAT (algebra)) Dear Dr. Greenie: Ah, yes, there it is... a simpler solution; easily performed within a minute! I know this is exactly what the GMAT Quantitative section is testing people on, the ability to see relationships between formulas and find the simplest way to solve problems. And, this is precisely the ability that I need to improve. Thanks again. Shawn Tokyo |
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