Skip to main content
Engineering LibreTexts

1.2: B.2- Right Triangle Trigonometry

  1. Last updated
  2. Save as PDF
  • Page ID
    70296

    • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    Section B.2Right Triangle Trigonometry

    A right triangle is a triangle containing a 90° angle.

    The side opposite to the right angle is called the hypotenuse.

    The other two angles add to 90° and are called complementary angles.

    The relationship between the sides and angles of a right triangle are given by the three basic trig relations which may be recalled with the mnemonic SOH-COH-TOA.

    \begin{align*} \sin\theta \amp= \frac{\textrm{opposite}}{\textrm{hypotenuse}} \amp \cos\theta \amp= \frac{\textrm{adjacent}}{\textrm{hypotenuse}} \amp \tan\theta \amp= \frac{\textrm{opposite}}{\textrm{adjacent}} \end{align*}

    and their inverses,

    \begin{align*} \theta \amp = \sin^{-1} \left(\frac{\textrm{opposite}}{\textrm{hypotenuse}}\right) \amp \theta \amp = \cos^{-1} \left(\frac{\textrm{adjacent}}{\textrm{hypotenuse}}\right) \amp \theta \amp = \tan^{-1} \left(\frac{\textrm{opposite}}{\textrm{adjacent}}\right) \end{align*}
    Facts.

    The following statements regarding the trig functions and triangles are always true, and remembering them will help you avoid errors.

    • \(\sin\text{,}\) \(\cos\) and \(\tan\) are functions of an angle and their values are unitless ratios of lengths.
    • The inverse trig functions are functions of unitless ratios and their results are angles.
    • The sine of an angle equals the cosine of its complement and vice-versa.
    • The sine and cosine of any angle is always a unitless number between -1 and 1, inclusive.
    • The sine, cosine, and tangent of angles between 0 and 90° are always positive.
    • The inverse trig functions of positive numbers will always yield angles between 0 and 90°
    • The legs of a right triangle are always shorter than the hypotenuse.
    • Only right triangles have a hypotenuse.
    Hints.

    Here are some useful tips for angle calculations

    • Take care that your calculator is set in degrees mode for this course.
    • Always work with angles between 0° and 90° and use positive arguments for the inverse trig functions.
    • Following this advice will avoid unwanted signs and incorrect directions caused because \(\dfrac{-a}{b} = \dfrac{a}{-b}\text{,}\) and \(\dfrac{a}{b} = \dfrac{-a}{-b}\) and the calculator can't distinguish between them.