Difference between revisions of "1976 AHSME Problems/Problem 27"
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Note that | Note that | ||
<cmath>x=\frac{\sqrt{\sqrt{5}+2}+\sqrt{\sqrt{5}-2}}{\sqrt{\sqrt{5}+1}}\cdot\frac{\sqrt{\sqrt{5}-1}}{\sqrt{\sqrt{5}-1}}=\frac{\sqrt{3+\sqrt{5}}+\sqrt{7-3\sqrt{5}}}{2}. \hspace{15mm} (\bigstar)</cmath> | <cmath>x=\frac{\sqrt{\sqrt{5}+2}+\sqrt{\sqrt{5}-2}}{\sqrt{\sqrt{5}+1}}\cdot\frac{\sqrt{\sqrt{5}-1}}{\sqrt{\sqrt{5}-1}}=\frac{\sqrt{3+\sqrt{5}}+\sqrt{7-3\sqrt{5}}}{2}. \hspace{15mm} (\bigstar)</cmath> | ||
− | Let <math>\sqrt{a}+\sqrt{b}=\sqrt{3+\sqrt{5}}</math> for some nonnegative rational numbers <math>a</math> and <math>b.</math> We square both sides of this equation, then simplify: | + | We find each term in the numerator separately: |
+ | <ol style="margin-left: 1.5em;"> | ||
+ | <li>Let <math>\sqrt{a}+\sqrt{b}=\sqrt{3+\sqrt{5}}</math> for some nonnegative rational numbers <math>a</math> and <math>b.</math> We square both sides of this equation, then simplify: | ||
<cmath>\begin{align*} | <cmath>\begin{align*} | ||
a+b+2\sqrt{ab}&=3+\sqrt{5} \\ | a+b+2\sqrt{ab}&=3+\sqrt{5} \\ | ||
Line 49: | Line 51: | ||
ab&=\frac54. | ab&=\frac54. | ||
\end{align*}</cmath> | \end{align*}</cmath> | ||
− | By inspection, we | + | By inspection, we get <math>\{a,b\}=\left\{\frac12,\frac52\right\}.</math> Alternatively, we conclude that <math>a</math> and <math>b</math> are the solutions to the quadratic equation <math>t^2-3t+\frac54=0</math> by Vieta's Formulas.<p> It follows that <cmath>\sqrt{3+\sqrt{5}}=\sqrt{\frac12}+\sqrt{\frac52}=\frac{\sqrt2}{2}+\frac{\sqrt{10}}{2}.</cmath></li><p> |
+ | <li>Similarly, It follows that <cmath>\sqrt{7-3\sqrt{5}}=\frac{3\sqrt2}{2}-\frac{\sqrt{10}}{2}.</cmath></li><p> | ||
+ | </ol> | ||
+ | Substituting these results into <math>(\bigstar),</math> we have <cmath>x=\frac{\left(\frac{\sqrt2}{2}+\frac{\sqrt{10}}{2}\right)+\left(\frac{3\sqrt2}{2}-\frac{\sqrt{10}}{2}\right)}{2}=\sqrt2.</cmath> | ||
+ | On the other hand, we have <cmath>y=\sqrt2-1</cmath> by the argument of either Solution 1 or Solution 2. | ||
+ | |||
+ | Finally, the answer is <math>N=x-y=\boxed{\textbf{(A) }1}.</math> | ||
+ | |||
+ | ~MRENTHUSIASM | ||
== See also == | == See also == |
Revision as of 02:30, 8 September 2021
Contents
Problem
If then equals
Solution 1
Let and Clearly, and are both positive.
Note that from which
On the other hand, note that Finally, the answer is
~Someonenumber011 (Fundamental Logic)
~MRENTHUSIASM (Reconstruction)
Solution 2
Let and Clearly, and are both positive.
Note that We find each term in the numerator separately:
- Let for some nonnegative rational numbers and We square both sides of this equation, then simplify:
It follows that
By inspection, we get Alternatively, we conclude that and are the solutions to the quadratic equation by Vieta's Formulas.
It follows that
- Similarly, It follows that
Substituting these results into we have On the other hand, we have by the argument of either Solution 1 or Solution 2.
Finally, the answer is
~MRENTHUSIASM
See also
1976 AHSME (Problems • Answer Key • Resources) | ||
Preceded by Problem 26 |
Followed by Problem 28 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30 | ||
All AHSME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.