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  1. Home
  2. 12th grade
  3. Geometry
  4. Exercise : Find the area of a triangle

Find the area of a triangle Geometry

Determine the area of the following triangles.

-

The area of the triangle is:

\dfrac{1}{2}\times\left(BC \times AH\right)

Or:

\dfrac{1}{2}\times\left[\left(BH+HC\right) \times AH\right]

Using the Pythagoras theorem, we have:

BH = \sqrt{5^2-3^2}= 4

Similarly, we have:

HC=4

The area of the triangle is:

\dfrac{1}{2}\times\left[\left(BH+HC\right) \times AH\right]=\dfrac{1}{2}\times\left[\left(4+4\right) \times 3\right]=12

The area of the triangle is 12.

-

The area of the triangle is:

\dfrac{1}{2}\times\left(BC \times AB\right)

We know that:

\tan{\widehat{C}} = \dfrac{AB}{BC}

And:

  • \widehat{C}= 30^\circ
  • \tan{30^\circ} = \dfrac{\sqrt{3}}{3}

Therefore:

\dfrac{\sqrt{3}}{3} = \dfrac{3}{BC}

BC = \dfrac{9}{\sqrt{3}} = 3\sqrt{3}

The area of the triangle is:

\dfrac{1}{2} \times AB \times BC = \dfrac{1}{2} \times3 \times 3{\sqrt{3}} = \dfrac{9{\sqrt{3}} }{2}

The area of the triangle is \dfrac{9{\sqrt{3}} }{2}.

-

The area of the triangle is:

\dfrac{1}{2}\times\left(BC \times AH\right)

Using the Pythagoras theorem, we have:

AH = \sqrt{5^2 - 3^2} = 4

Therefore:

S = \dfrac{1}{2} \times 6 \times 4 = 12

The area of this triangle is 12.

-

The area of the triangle is:

S = \dfrac{1}{2} \times BC \times AH

Using the Pythagoras theorem, we have:

AH = \sqrt{4^2 - 2^2} = \sqrt{12}= 2\sqrt{3}

-

Therefore:

S = \dfrac{1}{2} \times 4 \times 2\sqrt{3} = 4\sqrt{3}

The area of this triangle is 4\sqrt{3}.

-

The area of the triangle is:

\dfrac{1}{2}\times\left(BC \times AH\right)

Or:

\dfrac{1}{2}\times\left[\left(BH+HC\right) \times AH\right]

Using the Pythagoras theorem, we have:

BH = \sqrt{5^2-3^2}= 4

Also:

HC=\sqrt{6^2-3^2} = \sqrt{27}

The area of the triangle is:

\dfrac{1}{2}\times\left[\left(BH+HC\right) \times AH\right]=\dfrac{1}{2}\times\left[\left(4+\sqrt{27}\right) \times 3\right]=\dfrac{3}{2}\left(4+\sqrt{27}\right)

The area of the triangle is \dfrac{3}{2}\left(4+\sqrt{27}\right).

-

The area of the triangle with sides a and b, and the included angle \theta, can be computed using the following formula:

A=\dfrac{1}{2}a.b \sin \left(\theta\right)

Therefore:

A=\dfrac{1}{2} \times 2 \times\sqrt2 \times \sin \left(45^\circ\right)

\dfrac{1}{2} \times 2 \times\sqrt2 \times \dfrac{\sqrt{2}}{2}

A=1

The area of the triangle is 1.

-

The area of the triangle is:

\dfrac{1}{2}\times\left(BC \times AH\right) or \dfrac{1}{2}\times\left[\left(BH+HC\right) \times AH\right]

We know that:

  • \tan{\widehat{C}} = \dfrac{AH}{BH}
  • \tan{\widehat{B}} = \dfrac{AH}{CH}

We also have:

  • \widehat{C}= 45^\circ
  • \widehat{B}= 45^\circ
  • \tan{45^\circ}= 1

We can write:

1 = \dfrac{2}{BH} \Rightarrow BH = 2

1 = \dfrac{2}{CH} \Rightarrow CH=2

The area of the triangle is:

\dfrac{1}{2} \times 2 \times \left(2+2\right)=4

The area of the triangle is 4.

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See also
  • Course : Triangles
  • Exercise : Use the law of cosine to determine lengths and angles
  • Exercise : Identify special triangles
  • Exercise : Use properties of special triangles to determine lengths, angles, heights
  • Exercise : Use congruence to determine measures
  • Exercise : Complete proofs involving triangles
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