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Trigonometry is the study of the relationships between the angles and sides in triangles. These relationships can be used to calculate different characteristics of triangles.

In a right triangle, the hypotenuse is always the longest side of the triangle and lies across from the right angle. The triangle's other two sides are called legs. If one of the acute angles in the triangle is labeled, the legs can be discussed relative to that angle. If $θ$ is labeled as shown, the side that forms the angle is the *adjacent* side and the side not touching the angle is the *opposite* side.

Change angles

By using the sides of a right triangle, six trigonometric functions of the acute angle $θ$ can be defined. The most common of these are sine, cosine and tangent.

$sin(θ)=hypopp $

$cos(θ)=hypadj $

$tan(θ)=adjopp $

Three other trigonometric functions are cosecant, secant and cotangent. They are defined like this.

$csc(θ)=opphyp $

$sec(θ)=adjhyp $

$cot(θ)=oppadj $

Evaluate the trigonometric functions for the angle $θ.$

Show Solution

To begin, we should label the triangle's sides. The longest side is the hypotenuse, the side next to $θ$ is the adjacent side and the side across from $θ$ is the opposite side.

We can now evaluate the trigonometric functions by using the definitions and the lengths of the sides in the triangle.

trig function | definition | value |
---|---|---|

$sin(θ)$ | $hypopp $ | $53 =0.6$ |

$cos(θ)$ | $hypadj $ | $54 =0.8$ |

$tan(θ)$ | $adjopp $ | $43 =0.75$ |

$csc(θ)$ | $opphyp $ | $35 ≈1.67$ |

$sec(θ)$ | $adjhyp $ | $45 =1.25$ |

$cot(θ)$ | $oppadj $ | $34 ≈1.33$ |

For a right triangle with legs $a$ and $b,$ and hypotenuse $c,$ the following is true.

$a_{2}+b_{2}=c_{2}$

For the right triangle, find $x$ and $y.$

Show Solution

To begin, we should note that since the given triangle is a right triangle, both the trigonometric ratios and the Pythagorean Theorem might be useful tools. The given information includes one angle and one side. Since two side lengths must be known to use the Pythagorean Theorem, we'll have to use a trigonometric ratio. We can label the sides of the triangle relative to the labeled angle.

Let's first find $x.$ Since the adjacent side is given, we must use $cosθ$ to determine $x.$$cos(θ)=hypadj $

$cos(30_{∘})=hyp4 $

Substitute$hyp=x$

$cos(30_{∘})=x4 $

MultEqn$LHS⋅x=RHS⋅x$

$x⋅cos(30_{∘})=4$

DivEqn$LHS/cos(30_{∘})=RHS/cos(30_{∘})$

$x=cos(30_{∘})4 $

UseCalcUse a calculator

$x=4.61880…$

RoundDecRound to ${\textstyle 2 \, \ifnumequal{2}{1}{\text{decimal}}{\text{decimals}}}$

$x≈4.62$

$a_{2}+b_{2}=c_{2}$

$4_{2}+b_{2}=(4.61880…)_{2}$

Substitute$b=y$

$4_{2}+y_{2}=(4.61880…)_{2}$

SubEqn$LHS−4_{2}=RHS−4_{2}$

$y_{2}=(4.61880…)_{2}−4_{2}$

UseCalcUse a calculator

$y_{2}=5.33333…$

SqrtEqn$LHS =RHS $

$y=±2.30940…$

$y>0$

$y=2.30940…$

RoundDecRound to ${\textstyle 2 \, \ifnumequal{2}{1}{\text{decimal}}{\text{decimals}}}$

$y≈2.31$

If the value of sine, cosine or tangent for a specific angle, $θ$, is known, it is possible to calculate the measure of the angle using inverse trigonometric functions. The inverse function of sine is called "inverse sine" and is written as $sin_{-1}.$

In the same way, inverse cosine ($cos_{-1}$) and inverse tangent ($tan_{-1}$) are the inverses of cosine and tangent, respectively.

Determine the measure of the angle $θ.$

Show Solution

In order to solve for $θ,$ we must use inverse trigonometric ratios. To begin, we'll label the sides of the triangle relative to $θ.$

Since the hypotenuse and the adjacent side are given, we'll use $cosθ.$

Since, $cos(θ)=6.15.4 ,$ it follows that $θ=cos_{-1}(6.15.4 ).$$θ=cos_{-1}(6.15.4 )$

UseCalcUse a calculator

$θ=27.71820…$

RoundDecRound to ${\textstyle 1 \, \ifnumequal{1}{1}{\text{decimal}}{\text{decimals}}}$

$θ≈27.7$

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