Small-angle approximation

The accuracy of the approximations can be seen below in Figure 1 and Figure 2. As the measure of the angle approaches zero, the difference between the approximation and the original function also approaches 0.

• 
  Figure 1. A comparison of the basic odd trigonometric functions to θ. It is seen that as the angle approaches 0 the approximations become better.
• 
  Figure 2. A comparison of cos θ to 1 − θ²/2. It is seen that as the angle approaches 0 the approximation becomes better.

The red section on the right, d, is the difference between the lengths of the hypotenuse, H, and the adjacent side, A. As is shown, H and A are almost the same length, meaning cos θ is close to 1 and θ²/2 helps trim the red away.

$${\displaystyle \cos {\theta }\approx 1-{\frac {\theta ^{2}}{2}}}$$

The opposite leg, O, is approximately equal to the length of the blue arc, s. Gathering facts from geometry, s = Aθ, from trigonometry, sin θ = O/H and tan θ = O/A, and from the picture, O ≈ s and H ≈ A leads to:

$${\displaystyle \sin \theta ={\frac {O}{H}}\approx {\frac {O}{A}}=\tan \theta ={\frac {O}{A}}\approx {\frac {s}{A}}={\frac {A\theta }{A}}=\theta .}$$

Simplifying leaves,

$${\displaystyle \sin \theta \approx \tan \theta \approx \theta .}$$

Using the squeeze theorem,[4] we can prove that

$${\displaystyle \lim _{\theta \to 0}{\frac {\sin(\theta )}{\theta }}=1,}$$

which is a formal restatement of the approximation ${\displaystyle \sin(\theta )\approx \theta }$ for small values of θ. A more careful application of the squeeze theorem proves that

$${\displaystyle \lim _{\theta \to 0}{\frac {\tan(\theta )}{\theta }}=1,}$$

from which we conclude that ${\displaystyle \tan(\theta )\approx \theta }$ for small values of θ. Finally, L'Hôpital's rule tells us that

$${\displaystyle \lim _{\theta \to 0}{\frac {\cos(\theta )-1}{\theta ^{2}}}=\lim _{\theta \to 0}{\frac {-\sin(\theta )}{2\theta }}=-{\frac {1}{2}},}$$

which rearranges to ${\textstyle \cos(\theta )\approx 1-{\frac {\theta ^{2}}{2}}}$ for small values of θ. Alternatively, we can use the double angle formula ${\displaystyle \cos 2A\equiv 1-2\sin ^{2}A}$. By letting ${\displaystyle \theta =2A}$, we get that ${\textstyle \cos \theta =1-2\sin ^{2}{\frac {\theta }{2}}\approx 1-{\frac {\theta ^{2}}{2}}}$.

The small-angle approximation for the sine function.

The Maclaurin expansion (the Taylor expansion about 0) of the relevant trigonometric function is[5]

$${\displaystyle {\begin{aligned}\sin \theta &=\sum _{n=0}^{\infty }{\frac {(-1)^{n}}{(2n+1)!}}\theta ^{2n+1}\\&=\theta -{\frac {\theta ^{3}}{3!}}+{\frac {\theta ^{5}}{5!}}-{\frac {\theta ^{7}}{7!}}+\cdots \end{aligned}}}$$

where θ is the angle in radians. In clearer terms,

$${\displaystyle \sin \theta =\theta -{\frac {\theta ^{3}}{6}}+{\frac {\theta ^{5}}{120}}-{\frac {\theta ^{7}}{5040}}+\cdots }$$

It is readily seen that the second most significant (third-order) term falls off as the cube of the first term; thus, even for a not-so-small argument such as 0.01, the value of the second most significant term is on the order of 0.000001, or 1/10000 the first term. One can thus safely approximate:

$${\displaystyle \sin \theta \approx \theta }$$

By extension, since the cosine of a small angle is very nearly 1, and the tangent is given by the sine divided by the cosine,

$${\displaystyle \tan \theta \approx \sin \theta \approx \theta ,}$$

In astronomy, the angular size or angle subtended by the image of a distant object is often only a few arcseconds, so it is well suited to the small angle approximation.[6] The linear size (D) is related to the angular size (X) and the distance from the observer (d) by the simple formula:

${\displaystyle D=X{\frac {d}{206\,265}}}$

where X is measured in arcseconds.

The number 206265 is approximately equal to the number of arcseconds in a circle (1296000), divided by 2π, or, the number of arcseconds in 1 radian.

The exact formula is

${\displaystyle D=d\tan \left(X{\frac {2\pi }{1\,296\,000}}\right)}$

and the above approximation follows when tan X is replaced by X.

The second-order cosine approximation is especially useful in calculating the potential energy of a pendulum, which can then be applied with a Lagrangian to find the indirect (energy) equation of motion.

When calculating the period of a simple pendulum, the small-angle approximation for sine is used to allow the resulting differential equation to be solved easily by comparison with the differential equation describing simple harmonic motion.

In optics, the small-angle approximations form the basis of the paraxial approximation.

The sine and tangent small-angle approximations are used in relation to the double-slit experiment or a diffraction grating to simplify equations, e.g. 'fringe spacing' = 'wavelength' × 'distance from slits to screen' ÷ 'slit separation'.[7]

The small-angle approximation also appears in structural mechanics, especially in stability and bifurcation analyses (mainly of axially-loaded columns ready to undergo buckling). This leads to significant simplifications, though at a cost in accuracy and insight into the true behavior.

The 1 in 60 rule used in air navigation has its basis in the small-angle approximation, plus the fact that one radian is approximately 60 degrees.

The formulas for addition and subtraction involving a small angle may be used for interpolating between trigonometric table values:

Example: sin(0.755)

$${\displaystyle {\begin{aligned}\sin(0.755)&=\sin(0.75+0.005)\\&\approx \sin(0.75)+(0.005)\cos(0.75)\\&\approx (0.6816)+(0.005)(0.7317)\\&\approx 0.6853.\end{aligned}}}$$

where the values for sin(0.75) and cos(0.75) are obtained from trigonometric table.