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Trigonometric identities
Pythagorean, double-angle and sum identities, verified by simplification.
Identities are equations true for every angle. The Pythagorean identity sin²x + cos²x = 1 is Pythagoras on the unit circle; the double-angle and sum formulas follow from it and rotation. To verify one, simplify the difference of the two sides to zero.
Ohatra: simplify sin(x)^2 + cos(x)^2
Simplify sin(x)^2 + cos(x)^2
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- \sin^{2}{\left(x \right)} + \cos^{2}{\left(x \right)}
Start from the expression.
- 1
Simplify.
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Symbols used here
Ratios of sides in a right triangle; coordinates on the unit circle.
Inequalities that allow equality; < and > exclude it.
Ratio of a circle's circumference to its diameter, 3.14159…
The usual name for an angle.
1/360 of a full turn. 180° = π radians.
The angle whose sine is the given value (and likewise arccos, arctan).
How to: Trigonometric identities
- Start from the expression.
- Simplify.
Questions people ask
Why radians instead of degrees?
A radian is the angle whose arc equals the radius, so it is a pure ratio rather than an arbitrary 1/360 of a turn. With radians the derivative of sin x is exactly cos x; with degrees a factor of π/180 appears everywhere.
Why does sin x = 1/2 have infinitely many solutions?
Sine repeats every full turn, and within one turn it reaches 1/2 twice (at π/6 and 5π/6). Add any whole number of turns to either and it is still true.
How do I remember the exact values?
Two triangles: the 45-45-90 with sides 1, 1, √2 and the 30-60-90 with sides 1, √3, 2. Every value for 30°, 45° and 60° is a ratio of those sides; symmetry gives the rest of the circle.
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Mbola maro ao Trigonometry
The unit circleTrigonometric equationsDegrees and radiansRight-triangle trigonometry (SOH-CAH-TOA)Law of sines and law of cosinesGraphs of sine, cosine and tangentInverse trigonometric functions