Manhattan Gmat problem
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√{3√{4*4*5} + 3(9-4√5)/(9-4√5)(9+4√5)}winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
√{12√5 + 3(9-4√5)/81-80?
√{12√5 + 27 - 12√5}
√27
3√3
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First, focus on the expression 3/(9+4√5). To simplify this, multiply the top and bottom by the conjugate of the bottom. The conjugate of an expression like this is just the same expression with the opposite sign in the middle. So in this case, it would be 9-4√5.winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
[3/(9+4√5)][(9-4√5)/(9-4√5)] = (27-12√5)/(81-36√5+36√5-80) = (27-12√5)/1 = 27-12√5. Plug this in to the original expression to get:
√{3√80+27-12√5}. Notice that 3√80 = 3√(16*5) = 12√5, so the entire expression simplifies to √27, which simplifies to [spoiler]3√3[/spoiler]
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√{3√80 + 3/(9+4√5)} = √{3 * 4√5 + 3/(9 + 4√5)} = √{12√5 + 3/(9 + 4√5)}winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
Now, 3/(9 + 4√5) = 3(9 - 4√5)/(9 + 4√5)(9 - 4√5) = 3(9 - 4√5)/(81 - 80) = 3(9 - 4√5)= 27 - 12√5
Therefore, √{3√80 + 3/(9+4√5)} = √{12√5 + 27 - 12√5} = √27 = [spoiler]3√3[/spoiler]
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If the answer choices are sufficiently different, we can ballpark the answer quickly:winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
√(3√80 + 3/(9+4√5))
≈ √( 3*√81 + smaller number/bigger number )
= √(3*9 + fraction)
= √(27 + fraction).
The fraction will have little effect on the result.
Thus, we can ignore it in our calculations:
√27 ≈ 3√3.
Last edited by GMATGuruNY on Thu Dec 15, 2011 6:53 pm, edited 2 times in total.
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Hi Mitch, I didn't understand how did you get to the step in bold red above.GMATGuruNY wrote:If the answer choices are sufficiently different, we can ballpark the answer quickly:winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
√(3√80 + 3/(9+4√5))
≈ √(3*9 + fraction)
≈ √27
≈ 3√7.
Just to correct √27 = 3√3
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√(3√80 + 3/(9+4√5))winniethepooh wrote:Hi Mitch, I didn't understand how did you get to the step in bold red above.GMATGuruNY wrote:If the answer choices are sufficiently different, we can ballpark the answer quickly:winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
√(3√80 + 3/(9+4√5)
≈ √(3*9 + fraction)
≈ √27
≈ 3√7.
Just to correct √27 = 3√3
to make the calculation much easier we replace √80 with √81 as they are approximately the same
√(3√81 + 3/(9+4√5))
√(3*9 + 3/(9+4√5))
4√5 = 2.23 * 4 = 9 approx.
3/(9+4√5) ≈ 3/(9+9) ≈ 1/6 ≈ 0.17
this is almost zero, so he neglected it
so the expression would be √(3*9 + 0.17) ≈ √3*9 ≈ 3√3
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Please revisit my post above, in which I fleshed out the explanation.winniethepooh wrote:Hi Mitch, I didn't understand how did you get to the step in bold red above.GMATGuruNY wrote:If the answer choices are sufficiently different, we can ballpark the answer quickly:winniethepooh wrote:√{3√80 + 3/(9+4√5)} = ?
√(3√80 + 3/(9+4√5))
≈ √(3*9 + fraction)
≈ √27
≈ 3√7.
Just to correct √27 = 3√3
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Wow Mitch, as always your explanations make such a lot of sense!!
That's really helpful in saving time!!
Thanks a lot.
That's really helpful in saving time!!
Thanks a lot.