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High School Math : Triangles

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Example Question #1 : Right Triangles

Rt_triangle_letters In this figure, side X=20 , Y=13 , and $c=90^\circ$ . What is the value of angle ?

Possible Answers:

Undefined

$6.5^\circ$

$90^\circ$

$49.46^\circ$

$40.54^\circ$

Correct answer:

$49.46^\circ$

Explanation:

Since $c=90^\circ$ , we know we are working with a right triangle.

That means that $\cos(\theta)=\frac{\text{adjacent}}{\text{hypotenuse}}$ .

In this problem, that would be:

$\cos(a)=\frac{\text{Y}}{\text{X}}$

Plug in our given values:

$\cos(a)=\frac{\text{13}}{\text{20}}$

$\cos(a)=0.65$

$a=\cos^{-1}(0.65)$

$a=49.46^\circ$

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Example Question #2 : Triangles

Let ABC be a right triangle with sides = 3 inches, = 4 inches, and = 5 inches. In degrees, what is the $\sin \Theta$ where $\Theta$ is the angle opposite of side ?

Possible Answers:

Correct answer:

Explanation:

3-4-5_triangle

We are looking for $\sin(\theta )$ . Remember the definition of sin in a right triangle is the length of the opposite side divided by the length of the hypotenuse.

So therefore, without figuring out $\Theta$ we can find

$\sin \Theta =\frac{3}{5}=.6$

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Example Question #71 : Trigonometry

Rt_triangle_letters

In this figure, if angle $c=90^\circ$ , side X=12 , and side Z=8 , what is the measure of angle ?

Possible Answers:

$\frac{2}{3}^\circ$

$90^\circ$

$48.2^\circ$

Undefined

$41.8^\circ$

Correct answer:

$41.8^\circ$

Explanation:

Since $c=90^\circ$ , we know we are working with a right triangle.

That means that $\sin(\theta)=\frac{\text{opposite}}{\text{hypotenuse}}$ .

In this problem, that would be:

$\sin(a)=\frac{\text{Z}}{\text{X}}$

Plug in our given values:

$\sin(a)=\frac{8}{12}$

$\sin(a)=\frac{2}{3}$

$a=\sin^{-1}(\frac{2}{3})$

$a=41.8^\circ$

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Example Question #61 : Pre Calculus

Rt_triangle_letters

In this figure, X=12 , $Y=6\sqrt{3}$ , and Z=6 . What is the value of angle ?

Possible Answers:

$45^\circ$

$60^\circ$

$90^\circ$

Undefined

$30^\circ$

Correct answer:

$30^\circ$

Explanation:

Notice that these sides fit the pattern of a 30:60:90 right triangle: $x:x\sqrt{3}:2x$ .

In this case, x=6 .

Since angle is opposite , it must be $30^\circ$ .

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Example Question #62 : Pre Calculus

A triangle has angles of $30^\circ:60^\circ:90^\circ$ . If the side opposite the $30^\circ$ angle is , what is the length of the side opposite $60^\circ$ ?

Possible Answers:

$7\sqrt{3}$

$\frac{7\sqrt{2}}{2}$

$14\sqrt{3}$

$7\sqrt{2}$

Correct answer:

$7\sqrt{3}$

Explanation:

The pattern for $30^\circ:60^\circ:90^\circ$ is that the sides will be $x:x\sqrt{3}:2x$ .

If the side opposite $30^\circ$ is , then the side opposite $60^\circ$ will be $7\sqrt{3}$ .

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Example Question #1 : Applying The Law Of Cosines

In $\Delta ABC$ , AB = 26 , BC = 32 , and AC = 23 . To the nearest tenth, what is $m \angle A$ ?

Possible Answers:

A triangle with these sidelengths cannot exist.

$81.3 ^{\circ }\textrm{ or } 98.7^{ \circ }$

$171.3^{ \circ }$

$81.3 ^{\circ }$

$98.7^{ \circ }$

Correct answer:

$81.3 ^{\circ }$

Explanation:

By the Triangle Inequality, this triangle can exist, since 23 + 26 > 32 .

By the Law of Cosines:

$\left ( BC\right )^{2} = \left ( AB\right )^{2} + \left ( AC\right )^{2} - 2 \cdot AB \cdot AC \cdot \cos m \angle A$

Substitute the sidelengths and solve for $\cos m \angle A$ :

$32^{2} = 26^{2} + 23^{2} - 2\cdot 26 \cdot 23 \cdot \cos m \angle A$

$1,024 =676 + 529 - 1,196 \cdot \cos m \angle A$

$1,024 =1,205 - 1,196 \cdot \cos m \angle A$

$-181 = - 1,196 \cdot \cos m \angle A$

$\cos m \angle A = 181 \div 1,196 \approx 0.1513$

$m \angle A \approx \cos^{-1} 0.1513 \approx 81.3 ^{\circ }$

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Example Question #1 : Triangles

A triangle has sides of length 12, 17, and 22. Of the measures of the three interior angles, which is the greatest of the three?

Possible Answers:

$86.4^{\circ }$

$130.1^{\circ }$

$93.6^{\circ }$

$97.2^{\circ }$

$119.2^{\circ }$

Correct answer:

$97.2^{\circ }$

Explanation:

We can apply the Law of Cosines to find the measure of this angle, which we will call :

$c^{2} = a^{2} + b^{2} -2ab \cos C$

The widest angle will be opposite the side of length 22, so we will set:

c=22 , a=12 , b=17

$22^{2} = 12^{2} + 17^{2} -2\cdot 12\cdot 17 \cos C$

$484 = 144 + 289 -408 \cos C$

$51= -408 \cos C$

$\cos C = -\frac{51}{408} = -0.125$

$C = \cos^{-1} 0.125 = 97.2^{\circ }$

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Example Question #2 : Applying The Law Of Cosines

In $\Delta ABC$ , $m \angle A = 66^{\circ }$ , AB = 26 , and AC = 23 . To the nearest tenth, what is ?

Possible Answers:

26.8

A triangle with these characteristics cannot exist.

47.9

112.4

41.1

Correct answer:

26.8

Explanation:

By the Law of Cosines:

$\left ( BC\right )^{2} = \left ( AB\right )^{2} + \left ( AC\right )^{2} - 2 \cdot AB \cdot AC \cdot \cos m \angle A$

or, equivalently,

$BC =\sqrt{ \left ( AB\right )^{2} + \left ( AC\right )^{2} - 2 \cdot AB \cdot AC \cdot \cos m \angle A}$

Substitute:

$BC =\sqrt{ 26^{2} +23^{2} - 2 \cdot 26 \cdot 23 \cdot \cos 66 ^{\circ }}$

$\approx \sqrt{ 676 +529 - 1,196 \cdot 0.4067}$

$\approx \sqrt{ 718.6} \approx 26.8$

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Example Question #1 : Applying The Law Of Sines

Rt_triangle_letters
In this figure, angle $a=30^\circ$ and side Z=15 . If angle $b=45^\circ$ , what is the length of side ?

Possible Answers:

7.5

$\frac{15\sqrt{2}}{2}$

$15\sqrt{2}$

$30\sqrt{2}$

Correct answer:

$15\sqrt{2}$

Explanation:

For this problem, use the law of sines:

$\frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}=\frac{\text{side opposite }c}{\sin(c)}$ .

In this case, we have values that we can plug in:

$\frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}$

$\frac{Z}{\sin(a)}=\frac{Y}{\sin{(b)}}$

$\frac{15}{\sin(30^\circ)}=\frac{Y}{\sin{(45^\circ)}}$

$\frac{15}{\frac{1}{2}}=\frac{Y}{\frac{\sqrt{2}}{2}}$

Cross multiply:

$15*\frac{\sqrt{2}}{2}=\frac{1}{2}Y$

Multiply both sides by :

$15\sqrt{2}=Y$

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Example Question #1 : Applying The Law Of Sines

Rt_triangle_letters

In this figure $a=22^\circ$ and $c=85^\circ$ . If X=30 , what is ?

Possible Answers:

0.374

0.997

20.1

11.28

30.09

Correct answer:

11.28

Explanation:

For this problem, use the law of sines:

$\frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }b}{\sin(b)}=\frac{\text{side opposite }c}{\sin(c)}$ .

In this case, we have values that we can plug in:

$\frac{\text{side opposite }a}{\sin(a)}=\frac{\text{side opposite }c}{\sin(c)}$

$\frac{Z}{\sin(a)}=\frac{X}{\sin{(c)}}$

$\frac{Z}{\sin(22^\circ)}=\frac{30}{\sin{85^\circ}}$

$\frac{Z}{0.375}=\frac{30}{0.997}$

$\frac{Z}{0.375}=30.09$

Z=30.09*0.375

Z=11.28

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High School Math : Triangles

Study concepts, example questions & explanations for High School Math

All High School Math Resources

Example Questions

Example Question #1 : Right Triangles

Example Question #2 : Triangles

Example Question #71 : Trigonometry

Example Question #61 : Pre Calculus

Example Question #62 : Pre Calculus

Example Question #1 : Applying The Law Of Cosines

Example Question #1 : Triangles

Example Question #2 : Applying The Law Of Cosines

Example Question #1 : Applying The Law Of Sines

Example Question #1 : Applying The Law Of Sines

All High School Math Resources

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