shyamprasadrao Posted June 15, 2011 Share Posted June 15, 2011 If x 1) -x 2) -1 3) 1 4) x 5) sqrt(x) OA A. I guess its D. Am I missing something here Quote Link to comment Share on other sites More sharing options...
800 Posted June 15, 2011 Share Posted June 15, 2011 I X I = positive. so, (-X . I X I) means (- (-). +) or positive. sqrt of positive is positive. IMO D. Quote Link to comment Share on other sites More sharing options...
Brent Hanneson Posted June 15, 2011 Share Posted June 15, 2011 If x A) -x B) -1 C) 1 D) x E) sqrt(x) A quick way to solve this question is to plug in a value for x. Since x So, the sqrt[-x (|x|)] = sqrt[5 (|-5|)] = sqrt[5(5)] = sqrt[25] = 5 Answer choice A is -x, so if x= -5, then -x = 5. Since our output above is 5, answer choice A is correct. Cheers, Brent Quote Link to comment Share on other sites More sharing options...
gnat Posted June 15, 2011 Share Posted June 15, 2011 wonderful......thanks Brent Quote Link to comment Share on other sites More sharing options...
800 Posted June 15, 2011 Share Posted June 15, 2011 What we are finding here ? sqrt (- x.|x|) = ? right ?. As x is negative - x is supposed to be positive . That means X. Besides, | - x| = x. so sqrt (-x.|x|)= sqrt (x.x) = sqrt (x^2) = x. The option is D. Please point out the my mistake. Quote Link to comment Share on other sites More sharing options...
shyamprasadrao Posted June 16, 2011 Author Share Posted June 16, 2011 Hey 800, You have presupposed that x is negative, i.e -x and you are calculation leads to x, which is positive in sign. Hence you need -(-x ).... Even simpler, lets assume, x to be -y , you have SQRT ( -(-y) . | -y | ) = SQRT (y^2 ) = Y.... But what is y ??? -x..... Hence answer is A. Thanks Brent for the explanation. Quote Link to comment Share on other sites More sharing options...
800 Posted June 16, 2011 Share Posted June 16, 2011 Hey 800, You have presupposed that x is negative, i.e -x and you are calculation leads to x, which is positive in sign. Hence you need -(-x ).... Even simpler, lets assume, x to be -y , you have SQRT ( -(-y) . | -y | ) = SQRT (y^2 ) = Y.... But what is y ??? -x..... Hence answer is A. Thanks Brent for the explanation. No. But I have calculated x with negative sign. Why should I add another negative sign after calculation. Please explain. I think in G-day I will commit same mistake if I am not clear. Quote Link to comment Share on other sites More sharing options...
800 Posted June 17, 2011 Share Posted June 17, 2011 Yes I got the point !!! I missed the basic of roots! Rule: a root can only have a negative value if 1. it is an odd root and 2. the base of the root is negative. Here I had to apply the second condition of the rule. Thanks Brent & shyamprasadrao to inspire me to review the basic. Cheers !!! Quote Link to comment Share on other sites More sharing options...
Red Bozo Posted March 14, 2012 Share Posted March 14, 2012 Hi, I just needed 1 clarification...as far as GMAT is concerned, is sqrt(100) = 10??? Or is sqrt(100) = +/- 10??? Quote Link to comment Share on other sites More sharing options...
Brent Hanneson Posted March 19, 2012 Share Posted March 19, 2012 The square root of any number (by definition) is always positive (or zero in the case of sqrt(0)=0). So, sqrt(100)=10, sqrt(49)=7, . . . etc. Cheers, Brent Quote Link to comment Share on other sites More sharing options...
HhjDubai Posted August 14, 2012 Share Posted August 14, 2012 Thanks Brent... Chose option d but agree with option a after ur explanation Quote Link to comment Share on other sites More sharing options...
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