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11 Jan 2011, 17:57
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If n is positive, is \(\sqrt {n} > 100\)?
(1) \(\sqrt {n-1} > 99\)
(2) \(\sqrt {n+1} > 101\)
(1) \(\sqrt {n-1} > 99\)
(2) \(\sqrt {n+1} > 101\)
11 Jan 2011, 18:08
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If n is positive, is \(\sqrt {n} > 100\)?
(1) \(\sqrt {n-1} > 99\):
Now, \(99^2+1\) is less than \(100^2\) (as \(100^2=(99+1)^2=99^2+2*99+1\)), so \(n\) may or may not be more than this value. Not sufficient.
(2) \(\sqrt {n+1} > 101\):
Sufficient.
Answer: B.
- Is \(\sqrt {n} > 100\)?
Is \(n>100^2\)?
(1) \(\sqrt {n-1} > 99\):
- \(n-1>99^2\);
\(n>99^2+1\)
Now, \(99^2+1\) is less than \(100^2\) (as \(100^2=(99+1)^2=99^2+2*99+1\)), so \(n\) may or may not be more than this value. Not sufficient.
(2) \(\sqrt {n+1} > 101\):
- \(n+1>101^2\);
\(n>101^2-1\);
\(n>(101-1)(101+1)\);
\(n>100*102\);
\(n>100*102>100^2\).
Sufficient.
Answer: B.
11 Jan 2011, 19:30
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tonebeeze
If n is positive, is \(\sqrt {n} > 100\)?
1. \(\sqrt {n-1} > 99\)
2. \(\sqrt {n+1} > 101\)
Can someone write out the algebra on this one, I just want to double check work. Thanks.
1. \(\sqrt {n-1} > 99\)
2. \(\sqrt {n+1} > 101\)
Can someone write out the algebra on this one, I just want to double check work. Thanks.
Even though you asked for algebra, let me point out that you don't really need algebra to solve this.
When you are considering big numbers, square roots of consecutive numbers differ by very little. e.g. \(\sqrt {10000} = 100\) and \(\sqrt {9999} = 99.995\)... Square roots of even small positive numbers differ by less than 1 e.g. \(\sqrt {1} = 1\) and \(\sqrt {2} = 1.414\) - Difference of just 0.414 \(\sqrt {3} = 1.732\) - Difference of just 0.318. The difference just keeps getting smaller and smaller.
So if \(\sqrt {n-1} > 99\), \(\sqrt {n}\) will be very close to \(\sqrt {n-1}\) and will also be greater than 99. But will it be greater than 100, we cannot say. So not sufficient.
If \(\sqrt {n+1} > 101\), then \(\sqrt {n}\) may not be greater than 101, but it will definitely be greater than 100 since between two consecutive integers, the square root difference will not reach 1 (as shown above).
