Difference between revisions of "User:Temperal/The Problem Solver's Resource1"
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<math>\cos (\pi-A) = -\cos A</math> | <math>\cos (\pi-A) = -\cos A</math> | ||
− | <math>\cos ( | + | <math>\sin (-A) = -\sin A</math> |
+ | |||
+ | <math>\cos (-A) = \cos A</math> | ||
<math>\tan (\pi+A) = \tan A</math> | <math>\tan (\pi+A) = \tan A</math> | ||
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<math>\tan (\pi/2-A)=\cot A</math> | <math>\tan (\pi/2-A)=\cot A</math> | ||
− | <math>\sec | + | <math>\sec (\pi/2-A)=\csc A</math> |
<math>\cos (\pi/2-A) = \sin A</math> | <math>\cos (\pi/2-A) = \sin A</math> | ||
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===Terminology and Notation=== | ===Terminology and Notation=== | ||
− | <math>\cot A=\frac{1}{\tan A}</math>, but <math>\cot A\ne\tan^{-1} A | + | <math>\cot A=\frac{1}{\tan A}</math>, but <math>\cot{A} \ne \tan^{-1} A</math>, the former being the reciprocal and the latter the inverse. |
− | <math>\csc A=\frac{1}{\sin A}</math>, but <math>\csc A\ne\sin^{-1} A | + | <math>\csc A=\frac{1}{\sin A}</math>, but <math>\csc{A} \ne \sin^{-1} A</math>. |
− | <math>\sec A=\frac{1}{\sin A}</math>, but <math>\sec A\ne\cos^{-1} A | + | <math>\sec A=\frac{1}{\sin A}</math>, but <math>\sec{A} \ne \cos^{-1} A</math>. |
Speaking of inverses: | Speaking of inverses: | ||
− | <math>\tan^{-1} A=\text{atan } A=\arctan A</math> | + | <math>\tan^{-1} A=\text{atan } A=\arctan{A}</math> |
− | <math>\cos^{-1} A=\text{acos } A=\arccos A</math> | + | <math>\cos^{-1} A=\text{acos } A=\arccos {A}</math> |
− | <math>\sin^{-1} A=\text{asin } A=\arcsin A</math> | + | <math>\sin^{-1} A=\text{asin } A=\arcsin{A}</math> |
===Sum of Angle Formulas=== | ===Sum of Angle Formulas=== | ||
<math>\sin (A \pm B)=\sin A \cos B \pm \cos A \sin B</math> | <math>\sin (A \pm B)=\sin A \cos B \pm \cos A \sin B</math> | ||
+ | |||
+ | If we can prove this one, the other ones can be derived easily using the "Basic Facts" identities above. In fact, we can simply prove the addition case, for plugging <math>A=-B</math> into the addition case gives the subtraction case. | ||
+ | |||
+ | As it turns out, there's quite a nice geometric proof of the addition case, though other methods, such as de Moivre's Theorem, exist. The following proof is taken from ''the Art of Problem Solving, Vol. 2'' and is due to Masakazu Nihei of Japan, who originally had it published in ''Mathematics & Informatics Quarterly'', Vol. 3, No. 2: | ||
+ | |||
+ | {{asy image|1=<asy> | ||
+ | pair A,B,C; | ||
+ | C=(0,0); | ||
+ | B=(10,0); | ||
+ | A=(6,4); | ||
+ | draw(A--B--C--cycle); | ||
+ | label("$A$",A,N); | ||
+ | label("$B$",B,E); | ||
+ | label("$C$",C,W); | ||
+ | draw(A--(6,0)); | ||
+ | label("$\beta$",A,(-1,-2)); | ||
+ | label("$\alpha$",A,(1,-2.5)); | ||
+ | label("$H$",(6,0),S); | ||
+ | draw((6,0)--(5.5,0)--(5.5,0.5)--(6,0.5)--cycle); | ||
+ | </asy>|2=right|3=Figure 1}} | ||
+ | |||
+ | We'll find <math>[ABC]</math> in two different ways: <math>\frac{1}{2}(AB)(AC)(\sin \angle BAC)</math> and <math>[ABH]+[ACH]</math>. We let <math>AH=1</math>. We have: | ||
+ | |||
+ | <math>[ABC]=[ABH]+[ACH]</math> | ||
+ | |||
+ | <math>\frac{1}{2}(AC)(AB)(\sin \angle BAC)=\frac{1}{2}(AH)(BH)+\frac{1}{2}(AH)(CH)</math> | ||
+ | |||
+ | <math>\frac{1}{2}\left(\frac{1}{\cos \beta}\right)\left(\frac{1}{\cos \alpha}\right)(\sin \angle BAC)=\frac{1}{2}(1)(\tan \alpha)(\tan \beta)</math> | ||
+ | |||
+ | <math>\frac{\sin (\alpha+\beta)}{\cos \alpha \cos \beta}=\frac{\sin \alpha}{\cos \alpha}+\frac{\sin \beta}{\cos \beta}</math> | ||
+ | |||
+ | <math>\sin(\alpha+\beta)=\sin \alpha \cos \beta +\sin \beta \cos \alpha</math> | ||
+ | |||
+ | <math>\mathbb{QED.}</math> | ||
+ | |||
+ | ---- | ||
<math>\cos (A \pm B)=\cos A \cos B \mp \sin A \sin B</math> | <math>\cos (A \pm B)=\cos A \cos B \mp \sin A \sin B</math> | ||
<math>\tan (A \pm B)=\frac{\tan A \pm \tan B}{1 \mp \tan A \tan B}</math> | <math>\tan (A \pm B)=\frac{\tan A \pm \tan B}{1 \mp \tan A \tan B}</math> | ||
+ | |||
+ | The following identities can be easily derived by plugging <math>A=B</math> into the above: | ||
<math>\sin2A=2\sin A \cos A</math> | <math>\sin2A=2\sin A \cos A</math> | ||
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for all <math>A</math>. | for all <math>A</math>. | ||
+ | |||
+ | These can be easily seen by going back to the unit circle and the definition of these trig functions. | ||
===Other Formulas=== | ===Other Formulas=== | ||
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In a triangle with sides <math>a</math>, <math>b</math>, and <math>c</math> opposite angles <math>A</math>, <math>B</math>, and <math>C</math>, respectively, | In a triangle with sides <math>a</math>, <math>b</math>, and <math>c</math> opposite angles <math>A</math>, <math>B</math>, and <math>C</math>, respectively, | ||
− | <math>c^2=a^2+b^2- | + | <math>c^2=a^2+b^2-2ab\cos C</math> |
+ | |||
+ | The proof is left as an exercise for the reader. (Hint: Draw a circle with one of the sides as a radius and use the power of a point theorem) | ||
− | |||
====Law of Sines==== | ====Law of Sines==== | ||
<math>\frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C}=2R</math> | <math>\frac{a}{\sin A}=\frac{b}{\sin B}=\frac{c}{\sin C}=2R</math> | ||
+ | |||
+ | where <math>R</math> is the radius of the circumcircle of <math>\triangle ABC</math> | ||
+ | |||
+ | Proof: In the diagram below, circle <math> O </math> [[circumscribe]]s triangle <math> ABC </math>. <math> OD </math> is [[perpendicular]] to <math> BC </math>. Since <math> \triangle ODB \cong \triangle ODC </math>, <math> BD = CD = \frac a2 </math> and <math> \angle BOD = \angle COD </math>. But <math> \angle BAC = 2\angle BOC </math> making <math> \angle BOD = \angle COD = \theta </math>. Therefore, we can use simple trig in [[right triangle]] <math> BOD </math> to find that | ||
+ | |||
+ | <center><math> \sin \theta = \frac{\frac a2}R \Leftrightarrow \frac a{\sin\theta} = 2R. </math> </center> | ||
+ | |||
+ | The same holds for <math>b</math> and <math>c</math>, thus establishing the identity. | ||
+ | |||
+ | <center>[[Image:Lawofsines.PNG]]</center> | ||
====Law of Tangents==== | ====Law of Tangents==== | ||
− | + | If <math>A</math> and <math>B</math> are angles in a triangle opposite sides <math>a</math> and <math>b</math> respectively, then | |
− | < | + | <cmath> \frac{a-b}{a+b}=\frac{\tan (A-B)/2}{\tan (A+B)/2} . </cmath> |
+ | |||
+ | The proof of this is less trivial than that of the law of sines and cosines, but still fairly easy: | ||
+ | Let <math>s</math> and <math>d</math> denote <math>(A+B)/2</math>, <math>(A-B)/2</math>, respectively. By the law of sines, | ||
+ | <cmath> \frac{a-b}{a+b} = \frac{\sin A - \sin B}{\sin A + \sin B} = \frac{ \sin(s+d) - \sin (s-d)}{\sin(s+d) + \sin(s-d)} . </cmath> | ||
+ | By the angle addition identities, | ||
+ | <cmath> \frac{\sin(s+d) - \sin(s-d)}{\sin(s+d) + \sin(s-d)} = \frac{2\cos s \sin d}{2\sin s \cos d} = \frac{\tan d}{\tan s} = \frac{\tan (A-B)/2}{\tan (A+B)/2} </cmath> | ||
+ | as desired. | ||
====Area of a Triangle==== | ====Area of a Triangle==== | ||
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<math>\frac 12ab\sin C</math> | <math>\frac 12ab\sin C</math> | ||
+ | |||
+ | This can be easily proven by the well-known formula <math>\frac{1}{2}ah_a</math> - considering one of the triangles which altitude <math>h_a</math> divides <math>\triangle ABC</math> into, we see that <math>h_a=b\sin C</math> and hence <math>[ABC]=\frac 12ab\sin C</math> as desired. | ||
[[User:Temperal/The Problem Solver's Resource|Back to intro]] | [[User:Temperal/The Problem Solver's Resource2|Continue to page 2]] | [[User:Temperal/The Problem Solver's Resource|Back to intro]] | [[User:Temperal/The Problem Solver's Resource2|Continue to page 2]] |
Latest revision as of 18:18, 21 January 2016
Introduction | Other Tips and Tricks | Methods of Proof | You are currently viewing page 1. |
Trigonometric Formulas
Note that all measurements are in radians.
Basic Facts
The above can all be seen clearly by examining the graphs or plotting on a unit circle - the reader can figure that out themselves.
Terminology and Notation
, but
, the former being the reciprocal and the latter the inverse.
, but
.
, but
.
Speaking of inverses:
Sum of Angle Formulas
If we can prove this one, the other ones can be derived easily using the "Basic Facts" identities above. In fact, we can simply prove the addition case, for plugging into the addition case gives the subtraction case.
As it turns out, there's quite a nice geometric proof of the addition case, though other methods, such as de Moivre's Theorem, exist. The following proof is taken from the Art of Problem Solving, Vol. 2 and is due to Masakazu Nihei of Japan, who originally had it published in Mathematics & Informatics Quarterly, Vol. 3, No. 2:
|
Figure 1 |
We'll find in two different ways:
and
. We let
. We have:
The following identities can be easily derived by plugging into the above:
or
or
Pythagorean identities
for all .
These can be easily seen by going back to the unit circle and the definition of these trig functions.
Other Formulas
Law of Cosines
In a triangle with sides ,
, and
opposite angles
,
, and
, respectively,
The proof is left as an exercise for the reader. (Hint: Draw a circle with one of the sides as a radius and use the power of a point theorem)
Law of Sines
where is the radius of the circumcircle of
Proof: In the diagram below, circle circumscribes triangle
.
is perpendicular to
. Since
,
and
. But
making
. Therefore, we can use simple trig in right triangle
to find that
![$\sin \theta = \frac{\frac a2}R \Leftrightarrow \frac a{\sin\theta} = 2R.$](http://latex.artofproblemsolving.com/0/3/f/03f858d898d88e6bb7665ea3514b180f2e8ebcf5.png)
The same holds for and
, thus establishing the identity.
Law of Tangents
If and
are angles in a triangle opposite sides
and
respectively, then
The proof of this is less trivial than that of the law of sines and cosines, but still fairly easy:
Let and
denote
,
, respectively. By the law of sines,
By the angle addition identities,
as desired.
Area of a Triangle
The area of a triangle can be found by
This can be easily proven by the well-known formula - considering one of the triangles which altitude
divides
into, we see that
and hence
as desired.