Difference between revisions of "Trigonometric identities"
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− | '''Trigonometric identities''' are equations involving | + | In [[trigonometry]], '''Trigonometric identities''' are equations involving trigonometric functions that are true for all input values. Trigonometric functions have an abundance of identities, of which only the most widely used are included in this article. |
== Pythagorean identities == | == Pythagorean identities == | ||
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*<math>\cos (180^{\circ} - x) = -\cos (x)</math> and *<math>\sec (180^{\circ} - x) = -\sec (x)</math> | *<math>\cos (180^{\circ} - x) = -\cos (x)</math> and *<math>\sec (180^{\circ} - x) = -\sec (x)</math> | ||
*<math>\tan (180^{\circ} - x) = -\tan (x)</math> and <math>\cot (180^{\circ} - x) = -\cot (x)</math> | *<math>\tan (180^{\circ} - x) = -\tan (x)</math> and <math>\cot (180^{\circ} - x) = -\cot (x)</math> | ||
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==See also== | ==See also== |
Revision as of 17:40, 30 May 2021
In trigonometry, Trigonometric identities are equations involving trigonometric functions that are true for all input values. Trigonometric functions have an abundance of identities, of which only the most widely used are included in this article.
Contents
[hide]Pythagorean identities
The Pythagorean identities state that
Using the ratio definition of trigonometry, we apply Pythagorean Theorem on our triangle above to get that . If we divide by , we get , which is just .
If one uses the unit circle definition of trigonometry, it's extremely easy to see that by the Pythagorean theorem.
To derive the other two Pythagorean identities, divide by either or and substitute the respective trigonometry in place of the ratios to obtain the desired result.
Angle addition identities
The trigonometric angle addition identities state the following identities:
There are many proofs of these identities. For the sake of brevity, we list only one here.
Euler's identity states that . We have that By looking at the real and imaginary parts, we derive the sine and cosine angle addition formulas.
To derive the tangent addition formula, we reduce tangent to sine and cosine, divide both numerator and denominator by , and simplify. as desired.
Double-angle identities
The trigonometric double-angle identities are easily derived from the angle addition formulas by just letting . Doing so yields:
Cosine double-angle identity
Here are two equally useful forms of the cosine double-angle identity. Both are derived via the Pythagorean identity on the cosine double-angle identity given above.
In addition, the following identities are useful in integration and in deriving the half-angle identities. They are a simple rearrangement of the two above.
Half-angle identities
The trigonometric half-angle identities state the following equalities:
The plus or minus is not saying that there are two answers, but that the sine of the angle depends on the quadrant that the angle is in.
Take the two expressions listed in the cosine double-angle section for and , and substitute instead of . Taking the square root then yields the desired half-angle identities for sine and cosine.
Sum-to-product identities
Product-to-sum identities
Coming soon
Other Identities
Here are some identities that are less significant than those above, but still useful.
Even-odd identities
The functions , , and are odd, while , , and are even. In other words, the six trigonometric functions satisfy the following equalities:
These are derived by the unit circle definitions of trigonometry. Making any angle negative is the same as reflecting it across the x-axis. This keeps its x-coordinate the same, but makes its y-coordinate negative. Thus, and .
Conversion identities
The following identities are useful when converting trigonometric functions.
- and
- and
- and
All of these can be proven via the angle addition identities.
Euler's identity
Euler's identity is a formula in complex analysis that connects complex exponentiation with trigonometry. It states that for any real number , where is Euler's constant and is the imaginary unit. Euler's identity is fundamental to the study of complex numbers and is widely considered among the most beautiful formulas in math.
Miscellaneous
These are the identities that do not contain enough substance to warrant a section of their own.
- and *
- and *
- and