Trigonometric equations involving a single trig function

Trigonometric equations are equations including trigonometric functions. If they have only such functions and constants, then the solution involves finding an unknown which is an argument to a trigonometric function.

Basic trigonometric equations

sin(x) = n

$n$	$\sin(x)=n$
$|n|<1$	${\begin{matrix}x=\alpha +2k\pi \\x=\pi -\alpha +2k\pi \\\alpha \in \left[-{\frac {\pi }{2}},{\frac {\pi }{2}}\right]\end{matrix}}$
$n=-1$	$x=-{\begin{matrix}{\frac {\pi }{2}}\end{matrix}}+2k\pi$
$n=0$	$x=k\pi$
$n=1$	$x={\begin{matrix}{\frac {\pi }{2}}\end{matrix}}+2k\pi$
$|n|>1$	$x\in \varnothing$

The equation $\sin(x)=n$ has solutions only when $n$ is within the interval $[-1,1]$ . If $n$ is within this interval, then we first find an $\alpha$ such that:

$\alpha =\arcsin(n)$

The solutions are then:

$x=\alpha +2k\pi$

$x=\pi -\alpha +2k\pi$

Where $k$ is an integer.

In the cases when $n$ equals 1, 0 or -1 these solutions have simpler forms which are summarized in the table on the right.

For example, to solve:

$\sin {\bigl (}{\tfrac {x}{2}}{\bigr )}={\frac {\sqrt {3}}{2}}$

First find $\alpha$ :

$\alpha =\arcsin {\bigl (}{\tfrac {\sqrt {3}}{2}}{\bigr )}={\frac {\pi }{3}}$

Then substitute in the formulae above:

${\frac {x}{2}}={\frac {\pi }{3}}+2k\pi$

${\frac {x}{2}}=\pi -{\frac {\pi }{3}}+2k\pi$

Solving these linear equations for $x$ gives the final answer:

$x={\frac {2\pi }{3}}(1+6k)$

$x={\frac {4\pi }{3}}(1+3k)$

Where $k$ is an integer.

cos(x) = n

$n$	$\cos(x)=n$
$|n|<1$	${\begin{matrix}x=\pm \alpha +2k\pi \\\alpha \in [0,\pi ]\end{matrix}}$
$n=-1$	$x=\pi +2k\pi$
$n=0$	$x={\begin{matrix}{\frac {\pi }{2}}\end{matrix}}+k\pi$
$n=1$	$x=2k\pi$
$|n|>1$	$x\in \varnothing$

Like the sine equation, an equation of the form $\cos(x)=n$ only has solutions when n is in the interval $[-1,1]$ . To solve such an equation we first find one angle $\alpha$ such that:

$\alpha =\arccos(n)$

Then the solutions for $x$ are:

$x=\pm \alpha +2k\pi$

Where $k$ is an integer.

Simpler cases with Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n} equal to 1, 0 or -1 are summarized in the table on the right.

tan(x) = n

$n!$	$\tan(x)=n$
General case	${\begin{matrix}x=\alpha +k\pi \\\alpha \in \left[-{\frac {\pi }{2}},{\frac {\pi }{2}}\right]\end{matrix}}$
$n=-1$	$x=-{\begin{matrix}{\frac {\pi }{4}}\end{matrix}}+k\pi$
$n=0$	$x=k\pi$
$n=1$	$x={\begin{matrix}{\frac {\pi }{4}}\end{matrix}}+k\pi$

An equation of the form $\tan(x)=n$ has solutions for any real $n$ . To find them we must first find an angle $\alpha$ such that:

$\alpha =\arctan(n)$

After finding $\alpha$ , the solutions for $x$ are:

$x=\alpha +k\pi$

When $n$ equals 1, 0 or -1 the solutions have simpler forms which are shown in the table on the right.

cot(x) = n

$n$	$\cot(x)=n$
General case	Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{matrix}x=\alpha+k\pi \\ \alpha\in\left[0;\pi\right]\end{matrix}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n=-1}	Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x=-\begin{matrix}\frac{3\pi}{4}\end{matrix}+k\pi}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n=0}	Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x=\begin{matrix}\frac{\pi}{2}\end{matrix}+k\pi}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n=1}	Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x=\begin{matrix}\frac{\pi}{4}\end{matrix}+k\pi}

The equation Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \cot(x)=n} has solutions for any real Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n} . To find them we must first find an angle Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \alpha} such that:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \alpha=\arccot(n)}

After finding Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \alpha} , the solutions for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} are:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x=\alpha+k\pi}

When Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n} equals 1, 0 or -1 the solutions have simpler forms which are shown in the table on the right.

csc(x) = n and sec(x) = n

The trigonometric equations Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \csc(x)=n} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sec(x)=n} can be solved by transforming them to other basic equations:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \csc(x)=n\ \Leftrightarrow\ \frac{1}{\sin(x)}=n\ \Leftrightarrow\ \sin(x)=\frac{1}{n}}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sec(x)=n\ \Leftrightarrow\ \frac{1}{\cos(x)}=n\ \Leftrightarrow\ \cos(x)=\frac{1}{n}}

Further examples

Generally, to solve trigonometric equations we must first transform them to a basic trigonometric equation using the [[../Trigonometric_Identities_Reference|trigonometric identities]]. This sections lists some common examples.

a sin(x)+b cos(x) = c

To solve this equation we will use the identity:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a\sin(x)+b\cos(x)=\sqrt{a^2+b^2}\sin(x+\alpha)}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \alpha=\begin{cases}\arctan\bigl(\frac{b}{a}\bigr), & \mbox{if } a>0 \\ \pi+\arctan\bigl(\frac{b}{a}\bigr), & \mbox{if } a<0 \end{cases}}

The equation becomes:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sqrt{a^2+b^2}\sin(x+\alpha)=c}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sin(x+\alpha)=\frac{c}{\sqrt{a^2+b^2}}}

This equation is of the form Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sin(x)=n} and can be solved with the formulae given above.

For example we will solve:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sin(3x)-\sqrt3\cos(3x)=-\sqrt3}

In this case we have:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle a=1,b=-\sqrt3}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sqrt{a^2+b^2}=\sqrt{1^2+\Big(-\sqrt3\Big)^2}=2}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \alpha=\arctan\Big(-\sqrt3\Big)=-\frac{\pi}{3}}

Apply the identity:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 2\sin\left(3x-\frac{\pi}{3}\right)=-\sqrt3}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sin\left(3x-\frac{\pi}{3}\right)=-\frac{\sqrt3}{2}}

So using the formulae for Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sin(x)=n} the solutions to the equation are:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 3x-\frac{\pi}{3}=-\frac{\pi}{3}+2k\pi\ \Leftrightarrow\ x=\frac{2k\pi}{3}}

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 3x-\frac{\pi}{3}=\pi+\frac{\pi}{3}+2k\pi\ \Leftrightarrow\ x=\frac{\pi}{9}(6k+5)}

Where Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle k} is an integer.

Resources

• Trigonometric equations involving a single trig function. Written notes created by Professor Esparza, UTSA.
• Trigonometric equations involving a single trig function continued. Written notes created by Professor Esparza, UTSA.

Licensing

Content obtained and/or adapted from:

• Solving Trigonometric Equations, Wikibooks: Trigonometry under a CC BY-SA license