Difference between revisions of "2021 Fall AMC 12A Problems/Problem 19"
MRENTHUSIASM (talk | contribs) (→Problem 19) |
MRENTHUSIASM (talk | contribs) (→Solution 2) |
||
Line 16: | Line 16: | ||
== Solution 2 == | == Solution 2 == | ||
− | For choice A, we have | + | For choice <math>\textbf{(A)},</math> we have |
<cmath> | <cmath> | ||
\begin{align*} | \begin{align*} | ||
− | \left| \sin x | + | \left| \sin x - \sin \left( x^2 \right) \right| |
− | & = \left| \sin 10^\circ | + | & = \left| \sin 10^\circ - \sin \left( \left( 10^2 \right)^\circ \right) \right| \\ |
& = \left| \sin 10^\circ - \sin 100^\circ \right| \\ | & = \left| \sin 10^\circ - \sin 100^\circ \right| \\ | ||
& = \left| \sin 10^\circ - \sin 80^\circ \right| \\ | & = \left| \sin 10^\circ - \sin 80^\circ \right| \\ | ||
Line 26: | Line 26: | ||
\end{align*} | \end{align*} | ||
</cmath> | </cmath> | ||
− | + | For choice <math>\textbf{(B)},</math> we have | |
− | For choice B, we have | ||
<cmath> | <cmath> | ||
\begin{align*} | \begin{align*} | ||
− | \left| \sin x | + | \left| \sin x - \sin \left( x^2 \right) \right| |
− | & = \left| \sin 13^\circ | + | & = \left| \sin 13^\circ - \sin \left( \left( 13^2 \right)^\circ \right) \right| \\ |
& = \left| \sin 13^\circ - \sin 169^\circ \right| \\ | & = \left| \sin 13^\circ - \sin 169^\circ \right| \\ | ||
& = \left| \sin 10^\circ - \sin 11^\circ \right| \\ | & = \left| \sin 10^\circ - \sin 11^\circ \right| \\ | ||
Line 37: | Line 36: | ||
\end{align*} | \end{align*} | ||
</cmath> | </cmath> | ||
− | + | For choice <math>\textbf{(C)},</math> we have | |
− | For choice C, we have | ||
<cmath> | <cmath> | ||
\begin{align*} | \begin{align*} | ||
− | \left| \sin x | + | \left| \sin x - \sin \left( x^2 \right) \right| |
− | & = \left| \sin 14^\circ | + | & = \left| \sin 14^\circ - \sin \left( \left( 14^2 \right)^\circ \right) \right| \\ |
& = \left| \sin 14^\circ - \sin 196^\circ \right| \\ | & = \left| \sin 14^\circ - \sin 196^\circ \right| \\ | ||
& = \left| \sin 14^\circ + \sin 16^\circ \right| \\ | & = \left| \sin 14^\circ + \sin 16^\circ \right| \\ | ||
Line 48: | Line 46: | ||
\end{align*} | \end{align*} | ||
</cmath> | </cmath> | ||
− | + | For choice <math>\textbf{(D)},</math> we have | |
− | For choice D, we have | ||
<cmath> | <cmath> | ||
\begin{align*} | \begin{align*} | ||
− | \left| \sin x | + | \left| \sin x - \sin \left( x^2 \right) \right| |
− | & = \left| \sin 19^\circ | + | & = \left| \sin 19^\circ - \sin \left( \left( 19^2 \right)^\circ \right) \right| \\ |
& = \left| \sin 19^\circ - \sin 361^\circ \right| \\ | & = \left| \sin 19^\circ - \sin 361^\circ \right| \\ | ||
& = \left| \sin 19^\circ - \sin 1^\circ \right| \\ | & = \left| \sin 19^\circ - \sin 1^\circ \right| \\ | ||
Line 59: | Line 56: | ||
\end{align*} | \end{align*} | ||
</cmath> | </cmath> | ||
− | + | For choice <math>\textbf{(E)},</math> we have | |
− | For choice E, we have | ||
<cmath> | <cmath> | ||
\begin{align*} | \begin{align*} | ||
− | \left| \sin x | + | \left| \sin x - \sin \left( x^2 \right) \right| |
− | & = \left| \sin 20^\circ | + | & = \left| \sin 20^\circ - \sin \left( \left( 20^2 \right)^\circ \right) \right| \\ |
& = \left| \sin 20^\circ - \sin 400^\circ \right| \\ | & = \left| \sin 20^\circ - \sin 400^\circ \right| \\ | ||
& = \left| \sin 20^\circ - \sin 40^\circ \right| \\ | & = \left| \sin 20^\circ - \sin 40^\circ \right| \\ | ||
Line 70: | Line 66: | ||
\end{align*} | \end{align*} | ||
</cmath> | </cmath> | ||
− | + | Therefore, the answer is <math>\boxed{\textbf{(B) }13},</math> as <math>\sin 11^\circ - \sin 10^\circ</math> is closest to <math>0.</math> | |
− | Therefore, the answer is <math>\boxed{\textbf{(B) }13}</math>. | ||
~Steven Chen (www.professorchenedu.com) | ~Steven Chen (www.professorchenedu.com) |
Revision as of 01:07, 26 November 2021
Contents
Problem
Let be the least real number greater than such that , where the arguments are in degrees. What is rounded up to the closest integer?
Solution 1
The smallest to make would require , but since needs to be greater than , these solutions are not valid.
The next smallest would require , or .
After a bit of guessing and checking, we find that , and , so the solution lies between and , making our answer
Note: One can also solve the quadratic and estimate the radical.
~kingofpineapplz
Solution 2
For choice we have For choice we have For choice we have For choice we have For choice we have Therefore, the answer is as is closest to
~Steven Chen (www.professorchenedu.com)
See Also
2021 Fall AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 18 |
Followed by Problem 20 |
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 | |
All AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.