Precalculus : Use trigonometric functions to calculate the area of a triangle

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #1 : Use Trigonometric Functions To Calculate The Area Of A Triangle

In triangle \displaystyle ABC, \displaystyle m\angle C = 30^\circ, \displaystyle BC = 40, and \displaystyle AC = 20.  Find the area of the triangle.

Possible Answers:

\displaystyle 400

\displaystyle 200

\displaystyle 800

\displaystyle 100

\displaystyle 1600

Correct answer:

\displaystyle 200

Explanation:

When given the lengths of two sides and the measure of the angle included by the two sides, the area formula is:

\displaystyle A = \frac {1}{2}ab\sin C \\

Plugging in the given values we are able to calculate the area.

\displaystyle A = \frac {1}{2}\cdot 40\cdot 20 \cdot \sin 30^\circ= 200

Example Question #2 : Use Trigonometric Functions To Calculate The Area Of A Triangle

Find the area of this triangle:

Tri area f

Possible Answers:

\displaystyle 136 un^ 2

\displaystyle 11.81 un^ 2

\displaystyle 68 un^ 2

\displaystyle 66.97 un^ 2

Correct answer:

\displaystyle 68 un^ 2

Explanation:

To find the area, use the formula associated with side, angle, side triangles which states,

 \displaystyle area = \frac{ 1}{2} a b \sin C

where \displaystyle a and \displaystyle b are side lengths and \displaystyle C is the included angle.

In our case,

\displaystyle a=8, b=17, C=90^\circ.

Plug the values into the area formula and solve.

\displaystyle area = \frac{ 1}{2} (8 )( 17 ) \cdot \sin( 90 ^ o ) = 68

Example Question #3 : Use Trigonometric Functions To Calculate The Area Of A Triangle

Find the area of this triangle:

Tri area d

Possible Answers:

\displaystyle 212.13 un^ 2

\displaystyle 82. 99 un ^ 2

\displaystyle 88.11 un^ 2

\displaystyle 53.96 un^ 2

Correct answer:

\displaystyle 82. 99 un ^ 2

Explanation:

Use the area formula to find area that is associated with the side angle side theorem for triangles.

 \displaystyle area= \frac{ 1}{2 } a b \sin C

where \displaystyle a and \displaystyle b are side lengths and \displaystyle C is the included angle.

Plugging these values into the formula above, we arrive at our final answer.

\displaystyle area = \frac{ 1}{2} (11) (23 ) \sin (41^o ) \approx 82.99

Example Question #4 : Use Trigonometric Functions To Calculate The Area Of A Triangle

Find the area of this triangle:

Tri area b

Possible Answers:

\displaystyle 25.38 un^ 2

\displaystyle 28 un^ 2

\displaystyle 11.83 un^ 2

\displaystyle 23.67 un^ 2

Correct answer:

\displaystyle 11.83 un^ 2

Explanation:

To solve, use the formula for area that is associated with the side angle side theorem for triangles,

\displaystyle area = \frac{ 1}{2} ab \sin C

where \displaystyle a and \displaystyle b are side lengths and \displaystyle C is the included angle.

Here we are using \displaystyle 25^o and not \displaystyle 90^o since that is the angle between \displaystyle 8 and \displaystyle 7.

Therefore,

\displaystyle a=8,b=7,C=25^\circ.

Plugging the above values into the area formula we arrive at our final answer.

\displaystyle area = \frac{ 1}{2} (8)(7) \sin (25^o ) \approx 11.83

Example Question #5 : Use Trigonometric Functions To Calculate The Area Of A Triangle

Find the area of this triangle:

Tri area a

Possible Answers:

\displaystyle 48.84 un^2

\displaystyle 133.91 un^2

\displaystyle 48.74 un^ 2

\displaystyle 66.95 un^ 2

Correct answer:

\displaystyle 48.74 un^ 2

Explanation:

Find the area using the formula associated the side angle side theorem of a triangle,

\displaystyle area = \frac{ 1}{2} ab \sin C

where \displaystyle a and \displaystyle b are side lengths and \displaystyle C is the included angle.

In this particular case,

\displaystyle a=15, b=19, C=20^\circ

therefore the area is found to be,

\displaystyle area = \frac {1}{2} (15)(19) \sin (20^o ) \approx 48.74.

Example Question #1 : Area Of A Triangle

Find the exact area of a triangle with side lengths of \displaystyle 2, \displaystyle 3, and \displaystyle 4.

Possible Answers:

\displaystyle \frac{\sqrt {135}}{4}

\displaystyle \frac{\sqrt{17}}{2}

\displaystyle \frac{\sqrt{135}}{8}

\displaystyle 1.5

\displaystyle 2.5

Correct answer:

\displaystyle \frac{\sqrt {135}}{4}

Explanation:

Use the Heron's Formula:

\displaystyle s=\frac{a+b+c}{2}

\displaystyle A=\sqrt{s(s-a)(s-b)(s-c)}

Solve for \displaystyle s.

\displaystyle s=\frac{a+b+c}{2}=\frac{2+3+4}{2}=\frac{9}{2}

Solve for the area.

\displaystyle A=\sqrt{s(s-a)(s-b)(s-c)}

\displaystyle A=\sqrt{\frac{9}{2}\left(\frac{9}{2}-2\right)\left(\frac{9}{2}-3\right)\left(\frac{9}{2}-4\right)}

\displaystyle A=\sqrt{\frac{9}{2}\left(\frac{5}{2}\right)\left(\frac{3}{2}\right)\left(\frac{1}{2}\right)}=\sqrt{\frac{135}{16}}=\frac{\sqrt{135}}{4}

Example Question #2 : Area Of A Triangle

What is the area of a triangle with side lengths \displaystyle 4 , \displaystyle 5, and \displaystyle 7 ?

 

Possible Answers:

\displaystyle area=4

\displaystyle area=16

\displaystyle area=16\sqrt{6}

\displaystyle area=4\sqrt{6}

\displaystyle area=\sqrt{92}

Correct answer:

\displaystyle area=4\sqrt{6}

Explanation:

We can solve this question using Heron's Formula. Heron's Formula states that:

The semiperimeter is

\displaystyle s=\frac{1}{2}(a+b+c)

where \displaystyle a\displaystyle b\displaystyle c are the sides of a triangle.

Then the area is

\displaystyle area=\sqrt{s(s-a)(s-b)(s-c)}

So if we plug in

\displaystyle s=\frac{1}{2}(4+5+7)

\displaystyle s=\frac{1}{2}(16)

\displaystyle s=8

So the area is

\displaystyle area=\sqrt{8(8-4)(8-5)(8-7)}

\displaystyle area=\sqrt{96}

\displaystyle area=4\sqrt{6}

Example Question #3 : Area Of A Triangle

What is the area of a triangle with sides \displaystyle 5\displaystyle 6, and \displaystyle 9 ?

Possible Answers:

\displaystyle area=100\sqrt{2}

\displaystyle area=200

\displaystyle area=10\sqrt{2}

\displaystyle area=10

\displaystyle area=20\sqrt{2}

Correct answer:

\displaystyle area=10\sqrt{2}

Explanation:

We can solve this question using Heron's Formula. Heron's Formula states that:

The semiperimeter is

\displaystyle s=\frac{1}{2}(a+b+c)

where \displaystyle a\displaystyle b\displaystyle c are the sides of a triangle.

Then the area is

\displaystyle area=\sqrt{s(s-a)(s-b)(s-c)}

So if we plug in

\displaystyle s=\frac{1}{2}(6+5+9)

\displaystyle s=\frac{1}{2}(20)

\displaystyle s=10

So the area is

\displaystyle area=\sqrt{10(10-6)(10-5)(10-9)}

\displaystyle area=\sqrt{200}

\displaystyle area=10\sqrt{2}

Example Question #1 : Use Trigonometric Functions To Calculate The Area Of A Triangle

What is the area of a triangle with side lengths of \displaystyle 3\displaystyle 4, and \displaystyle 5 ?

Possible Answers:

\displaystyle area=6

\displaystyle area= 5

\displaystyle area= 7

\displaystyle area= 7.5

\displaystyle area= 12

Correct answer:

\displaystyle area=6

Explanation:

We can solve this question using Heron's Formula. Heron's Formula states that:

The semiperimeter is

\displaystyle s=\frac{1}{2}(a+b+c)

where \displaystyle a\displaystyle b\displaystyle c are the sides of a triangle.

Then the area is

\displaystyle area=\sqrt{s(s-a)(s-b)(s-c)}

So if we plug in

\displaystyle s=\frac{1}{2}(3+4+5)

\displaystyle s=\frac{1}{2}(12)

\displaystyle s=6

So the area is

\displaystyle area=\sqrt{6(6-3)(6-4)(6-5)}

\displaystyle area=\sqrt{36}

\displaystyle area=6

Example Question #10 : Use Trigonometric Functions To Calculate The Area Of A Triangle

What is the area of a triangle with side lengths \displaystyle 5\displaystyle 12, and \displaystyle 13 ?

Possible Answers:

\displaystyle area=20

\displaystyle area=33

\displaystyle area=32.5

\displaystyle area=900

\displaystyle area=30

Correct answer:

\displaystyle area=30

Explanation:

We can solve this question using Heron's Formula. Heron's Formula states that:

The semiperimeter is

\displaystyle s=\frac{1}{2}(a+b+c)

where \displaystyle a\displaystyle b\displaystyle c are the sides of a triangle.

Then the area is

\displaystyle area=\sqrt{s(s-a)(s-b)(s-c)}

So if we plug in

\displaystyle s=\frac{1}{2}(5+12+13)

\displaystyle s=\frac{1}{2}(30)

\displaystyle s=15

So the area is

\displaystyle area=\sqrt{15(15-5)(15-12)(15-13)}

\displaystyle area=\sqrt{900}

\displaystyle area=30

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