How to Find Missing Sides and Angles of a Right Triangle

How to Find Missing Sides and Angles of a Right Triangle

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In a right triangle, when one side and an angle are given we can find a missing side.

Trigonometric Ratios

The following trigonometric ratios can be used to determine the length of the missing side in a right triangle.

\(Sin \ θ \ = \ \frac{Opposite \ Side}{Hypotenuse}\)

\(Cos \ θ \ = \ \frac{Adjacent \ Side}{Hypotenuse}\)

\(tan \ θ \ = \ \frac{Opposite \ Side}{Adjacent \ Side}\)

\(Cot \ θ \ = \ \frac{Adjacent \ Side }{Opposite \ Side}\)

\(Cosec \ θ \ = \ \frac{Hypotenuse}{Opposite \ Side}\)

\(Sec \ θ \ = \ \frac{Hypotenuse}{Adjacent \ Side \ to}\)

Example

Find the value of \(x\).

Missing Sides and Angles of a Right Triangle

Solution

  • Hypotenuse Side \(= \ AC\)
  • Opposite Side \(= \ AB \ = \ x\)
  • Adjacent Side \(= \ BC \ = \ 7\)
  • \(θ \ = \ 30\)

The trigonometric ratio \(tan\) involves the adjacent and opposing sides.

\(tan(θ) \ = \ \frac{Opposite \ Side}{Adjacent \ Side} \ = \ \frac{AB}{BC}\) \(⇒ \ \frac{\sqrt{3}}{3} \ = \ \frac{x}{7} \ ⇒ \ x \ = \ \frac{7\sqrt{3}}{3} \ ≈ \ 4.04\)

So, \(4.04\) is the measurement of the missing side.

Free printable Worksheets

Exercises for Finding the Missing Sides and Angles of a Right Triangle

1) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle1

2) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle2

3) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle3

4) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle4

5) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle5

6) Find the answer: \(sin \ x \ = \ ? \ , \ cos \ x \ = \ ?\)

Missing Sides and Angles of a Right Triangle6

7) Find the answer: \(cos \ x \ = \ ? \ , \ tan \ x \ = \ ?\)

Missing Sides and Angles of a Right Triangle7

8) Find the answer: \(cot \ x \ = \ ? \ , \ sin \ x \ = \ ?\)

Missing Sides and Angles of a Right Triangle8

9) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle9

10) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle10

 

1) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle1

\(\color{red}{tan \ 45° \ = \ 1 \ = \ \frac{AB}{BC} \ = \ \frac{x}{7} \ ⇒ \ x \ = \ \frac{\sqrt{2}}{2} \times 7 \ = \ \frac{7\sqrt{2}}{2}}\)

2) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle2

\(\color{red}{tan \ x \ = \ \frac{AB}{BC} \ = \ \frac{5}{10} \ = \ \frac{1}{2} \ ⇒ \ x \ ≈ \ 26.565°}\)

3) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle3

\(\color{red}{tan \ 60° \ = \ \sqrt{3} \ = \ \frac{AB}{BC} \ = \ \frac{6}{x} \ ⇒ \ x \ = \ \frac{6}{\sqrt{3}} \ = \ 2\sqrt{3}}\)

4) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle4

\(\color{red}{tan \ 60° \ = \ \sqrt{3} \ = \ \frac{BC}{AB} \ = \ \frac{x}{11} \ ⇒ \ x \ = \ \sqrt{3} \times 11 \ = \ 11\sqrt{3}}\)

5) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle5

\(\color{red}{tan \ x \ = \ \frac{BC}{AB} \ = \ \frac{4\sqrt{3}}{4} \ = \ \sqrt{3} \ ⇒ \ x \ = \ 60°}\)

6) Find the answer: \(sin \ x \ = \ ? \ , \ cos \ x \ = \ ?\)

Missing Sides and Angles of a Right Triangle6

\(\color{red}{AC \ = \ \sqrt{3^2 \ + \ 4^2} \ = \ 5}\)
\(\color{red}{sin \ x \ = \ \frac{BC}{AC} \ = \ \frac{4}{5}}\)
\(\color{red}{cos \ x \ = \ \frac{AB}{AC} \ = \ \frac{3}{5}}\)

7) Find the answer: \(cos \ x \ = \ ? \ , \ tan \ x \ = \ ?\)

Missing Sides and Angles of a Right Triangle7

\(\color{red}{BC \ = \ \sqrt{10^2 \ - \ 8^2} \ = \ \sqrt{36} \ = \ 6}\)
\(\color{red}{cos \ x \ = \ \frac{BC}{AC} \ = \ \frac{6}{10} \ = \ \frac{3}{5}}\)
\(\color{red}{tan \ x \ = \ \frac{AB}{AC} \ = \ \frac{8}{6} \ = \ \frac{4}{3}}\)

8) Find the answer: \(cot \ x \ = \ ? \ , \ sin \ x \ = \ ?\)

Missing Sides and Angles of a Right Triangle8

\(\color{red}{AB \ = \ \sqrt{15^2 \ - \ 12^2} \ = \ \sqrt{81} \ = \ 9}\)
\(\color{red}{cot \ x \ = \ \frac{BC}{AB} \ = \ \frac{12}{9} \ = \ \frac{4}{3}}\)
\(\color{red}{sin \ x \ = \ \frac{AB}{AC} \ = \ \frac{9}{15} \ = \ \frac{3}{5}}\)

9) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle9

\(\color{red}{cos \ 45° \ = \ \frac{\sqrt{2}}{2} \ = \ \frac{BC}{AC} \ = \ \frac{9}{x} \ ⇒ \ x \ = \ 9 \times \frac{2}{\sqrt{2}} \ = \ \frac{\sqrt{2}}{9}}\)

10) Find the answer: \(x \ = \ ?\)

Missing Sides and Angles of a Right Triangle10

\(\color{red}{cos \ 49° \ ≈ \ 0.656 \ = \ \frac{BC}{AC} \ = \ \frac{9}{x} \ ⇒ \ x \ = \ 9 \times \frac{1}{0.656} \ ≈ \ 13.719}\)

Missing Sides and Angles of a Right Triangle Practice Quiz