Intrigonometry, it is common to usemnemonics to help remembertrigonometric identities and the relationships between the varioustrigonometric functions.

Thesine,cosine, andtangent ratios in a right triangle can be remembered by representing them as strings of letters, for instance SOH-CAH-TOA in English:

Sine =Opposite ÷Hypotenuse

Cosine =Adjacent ÷Hypotenuse

Tangent =Opposite ÷Adjacent

One way to remember the letters is to sound them out phonetically (i.e./ˌsoʊkəˈtoʊə/SOH-kə-TOH-ə, similar toKrakatoa).^[1]

Another method is to expand the letters into a sentence, such as "Some Old Horses Chew Apples Happily Throughout Old Age", "Some Old Hippy Caught Another Hippy Tripping On Acid", or "Studying Our Homework Can Always Help To Obtain Achievement". The order may be switched, as in "Tommy On A Ship Of His Caught A Herring" (tangent, sine, cosine) or "The Old Army Colonel And His Son Often Hiccup" (tangent, cosine, sine) or "Come And Have Some Oranges Help To Overcome Amnesia" (cosine, sine, tangent).^[2][3] Communities in Chinese circles may choose to remember it as TOA-CAH-SOH, which also means 'big-footed woman' (Chinese:大腳嫂;Pe̍h-ōe-jī:tōa-kha-só) inHokkien.^{[citation needed]}

An alternate way to remember the letters for Sin, Cos, and Tan is to memorize the syllables Oh, Ah, Oh-Ah (i.e./oʊəˈoʊ.ə/) for O/H, A/H, O/A.^[4] Longer mnemonics for these letters include "Oscar Has A Hold On Angie" and "Oscar Had A Heap of Apples."^[2]

All StudentsTakeCalculus is amnemonic for the sign of eachtrigonometric functions in eachquadrant of the plane. The letters ASTC signify which of the trigonometric functions are positive, starting in the top right 1st quadrant and movingcounterclockwise through quadrants 2 to 4.^[5]

• Quadrant 1 (angles from 0 to 90 degrees, or 0 to π/2 radians):All trigonometric functions are positive in this quadrant.
• Quadrant 2 (angles from 90 to 180 degrees, or π/2 to π radians):Sine and cosecant functions are positive in this quadrant.
• Quadrant 3 (angles from 180 to 270 degrees, or π to 3π/2 radians):Tangent and cotangent functions are positive in this quadrant.
• Quadrant 4 (angles from 270 to 360 degrees, or 3π/2 to 2π radians):Cosine and secant functions are positive in this quadrant.

Other mnemonics include:

• All StationsToCentral^[6]
• All SillyTomCats^[6]
• AddSugarToCoffee^[6]
• AllScienceTeachers (are)Crazy^[7]
• ASmartTrigClass^[8]
• AllSchoolsTortureChildren^[5]
• AwfulStinkyTrigCourse^[5]

Other easy-to-remember mnemonics are theACTS andCAST laws. These have the disadvantages of not going sequentially from quadrants 1 to 4 and not reinforcing the numbering convention of the quadrants.

• CAST still goes counterclockwise but starts in quadrant 4 going through quadrants 4, 1, 2, then 3.
• ACTS still starts in quadrant 1 but goes clockwise going through quadrants 1, 4, 3, then 2.

Sines and cosines of common angles 0°, 30°, 45°, 60° and 90° follow the pattern${\displaystyle \frac{\sqrt{n}}{2}}$ withn = 0, 1, ..., 4 for sine andn = 4, 3, ..., 0 for cosine, respectively:^[9]

${\displaystyle \theta }$	${\displaystyle \sin \theta }$	${\displaystyle \cos \theta }$	${\displaystyle \tan \theta =\sin \theta /\cos \theta }$
0° = 0 radians	${\displaystyle \frac{\sqrt{\mathbf{0}}}{2}=0}$	${\displaystyle \frac{\sqrt{\mathbf{4}}}{2}=1}$	${\displaystyle 0/1=0}$
30° =⁠π/6⁠ radians	${\displaystyle \frac{\sqrt{\mathbf{1}}}{2}=\frac{1}{2}}$	${\displaystyle \frac{\sqrt{\mathbf{3}}}{2}}$	${\displaystyle \frac{1}{2}/\frac{\sqrt{3}}{2}=\frac{1}{\sqrt{3}}}$
45° =⁠π/4⁠ radians	${\displaystyle \frac{\sqrt{\mathbf{2}}}{2}=\frac{1}{\sqrt{2}}}$	${\displaystyle \frac{\sqrt{\mathbf{2}}}{2}=\frac{1}{\sqrt{2}}}$	${\displaystyle \frac{1}{\sqrt{2}}/\frac{1}{\sqrt{2}}=1}$
60° =⁠π/3⁠ radians	${\displaystyle \frac{\sqrt{\mathbf{3}}}{2}}$	${\displaystyle \frac{\sqrt{\mathbf{1}}}{2}=\frac{1}{2}}$	${\displaystyle \frac{\sqrt{3}}{2}/\frac{1}{2}=\sqrt{3}}$
90° =⁠π/2⁠ radians	${\displaystyle \frac{\sqrt{\mathbf{4}}}{2}=1}$	${\displaystyle \frac{\sqrt{\mathbf{0}}}{2}=0}$	${\displaystyle 1/0=}$ undefined

Another mnemonic permits all of the basic identities to be read off quickly. The hexagonal chart can be constructed with a little thought:^[10]

1. Draw three triangles pointing down, touching at a single point. This resembles afallout sheltertrefoil.
2. Write a 1 in the middle where the three triangles touch
3. Write the functions without "co" on the three left outer vertices (from top to bottom:sine,tangent,secant)
4. Write the co-functions on the corresponding three right outer vertices (cosine,cotangent,cosecant)

Starting at any vertex of the resulting hexagon:

Identity	Example(s)	Illustration
The starting vertex equals one over the opposite vertex.	${\displaystyle \sin A=\frac{1}{\csc A}}$
	${\displaystyle \tan A=\frac{1}{\cot A}}$
	${\displaystyle \cos A=\frac{1}{\sec A}}$
Going either clockwise or counter-clockwise, the starting vertex equals the next vertex divided by the vertex after that.	${\displaystyle \sin A=\frac{\cos A}{\cot A}=\frac{\tan A}{\sec A}}$
	${\displaystyle \cos A=\frac{\cot A}{\csc A}=\frac{\sin A}{\tan A}}$
	${\displaystyle \cot A=\frac{\csc A}{\sec A}=\frac{\cos A}{\sin A}}$
	${\displaystyle \csc A=\frac{\sec A}{\tan A}=\frac{\cot A}{\cos A}}$
	${\displaystyle \sec A=\frac{\tan A}{\sin A}=\frac{\csc A}{\cot A}}$
	${\displaystyle \tan A=\frac{\sin A}{\cos A}=\frac{\sec A}{\csc A}}$
The starting corner equals the product of its two nearest neighbors.	${\displaystyle \sin A=\cos A\cdot \tan A}$
The sum of the squares of the two items at the top of a triangle equals the square of the item at the bottom. These are thetrigonometric Pythagorean identities.	${\displaystyle {\sin }^{2}A+{\cos }^{2}A=1^2=1}$
	${\displaystyle 1+{\cot }^{2}A={\csc }^{2}A}$
	${\displaystyle {\tan }^{2}A+1={\sec }^{2}A}$

Aside from the last bullet, the specific values for each identity are summarized in this table:

Starting function	... equals⁠1/opposite⁠	... equals⁠first/second⁠ clockwise	... equals⁠first/second⁠ counter-clockwise/anticlockwise	... equals the product of two nearest neighbors
${\displaystyle \tan A}$	${\displaystyle =\frac{1}{\cot A}}$	${\displaystyle =\frac{\sin A}{\cos A}}$	${\displaystyle =\frac{\sec A}{\csc A}}$	${\displaystyle =\sin A\cdot \sec A}$
${\displaystyle \sin A}$	${\displaystyle =\frac{1}{\csc A}}$	${\displaystyle =\frac{\cos A}{\cot A}}$	${\displaystyle =\frac{\tan A}{\sec A}}$	${\displaystyle =\cos A\cdot \tan A}$
${\displaystyle \cos A}$	${\displaystyle =\frac{1}{\sec A}}$	${\displaystyle =\frac{\cot A}{\csc A}}$	${\displaystyle =\frac{\sin A}{\tan A}}$	${\displaystyle =\sin A\cdot \cot A}$
${\displaystyle \cot A}$	${\displaystyle =\frac{1}{\tan A}}$	${\displaystyle =\frac{\csc A}{\sec A}}$	${\displaystyle =\frac{\cos A}{\sin A}}$	${\displaystyle =\cos A\cdot \csc A}$
${\displaystyle \csc A}$	${\displaystyle =\frac{1}{\sin A}}$	${\displaystyle =\frac{\sec A}{\tan A}}$	${\displaystyle =\frac{\cot A}{\cos A}}$	${\displaystyle =\cot A\cdot \sec A}$
${\displaystyle \sec A}$	${\displaystyle =\frac{1}{\cos A}}$	${\displaystyle =\frac{\tan A}{\sin A}}$	${\displaystyle =\frac{\csc A}{\cot A}}$	${\displaystyle =\csc A\cdot \tan A}$

• List of trigonometric identities

• Trigonometry
• Science mnemonics

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