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Geometry Level 2

In Triangle ABC ,AB= 8 cm ,BC= 12 cm *An. ABC=150 . Then,Find the Area of *Triangle ABC ...


The answer is 24.

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7 solutions

Kho Yen Hong
Jun 22, 2014

direct application of A r e a = 1 2 a b s i n θ Area=\frac{1}{2}absin\theta

1 2 8 × 12 × s i n 150 = 24 \frac{1}{2}8\times 12\times sin150 = 24

Give Angle ABC = 150, its supplement, Angle ABK, must be 30. That makes Triangle ABK a 30:60:90 and thus side AK must be half the hypotenuse because it is opposite the 30 degree angle and all 30: 60: 90 triangles have sides in proportion a : a(sqrt { 3 }): 2a. As side AK is also the height of Triangle ABC, we have Area = 1/2 4 12 = 24.

@Michael Goldenberg Sir do you know the formula .5a b sin theta?That is an easier way of solving this.

Mardokay Mosazghi - 6 years, 11 months ago
Awnon Bhowmik
Jul 11, 2014

Area of a triangle = 0.5 a b*sin x where x is the included angle between the sides a and b.

Easy problem

Why can't i use the cos rule Assuming that area=AB 2 +BC 2+ 2 ABxBC Cos 150 ???

Moheb Mohi - 6 years, 10 months ago

Moheb Mohi, that formula gives you the squared length of the side AC, not the area.

Mulindi Trevor - 6 years, 10 months ago
Akash Deep
Jun 30, 2014

actually the question should not have included the construction of perpendicular, it would be more elementary

@akash i agree

Mardokay Mosazghi - 6 years, 11 months ago

I'll keep that in mind...Thanks

Archiet Dev - 6 years, 11 months ago

Log in to reply

Why can't i use the cos rule Assuming that area=AB 2 +BC 2+ 2 ABxBC Cos 150 ???

Moheb Mohi - 6 years, 10 months ago

1/2 X 8 X 12 X sin 150 = 4 x 12 x 1/2 = 24

Isaac Jiménez
Jun 19, 2014

The angle A B C = 150 ° \measuredangle ABC=150° , we can assume that A K AK is an altitude of the triangule A B C ABC . So A B K = 30 ° \measuredangle ABK=30° and B A K = 60 ° \measuredangle BAK=60° ; so the triangle A K B AKB is half of an equilateral triangle, so 2 A K = A B 2AK=AB so A K = 4 AK=4 . So the area is 24 \boxed { 24 }

If the angle is 150º, your complement is 30º. Thus, sin 30º= 1/2. Then AK is x, sin 30º = x/8, 1/2 = x/8 --> 2x = 8 --> x = 4

If AK = 4, hence, the area of triangle ABC is height versus base divided by 2. AK is height and BC is base, then area of triangle is: (4 x 12) / 2 --> 48/2 = 24. The answer is 24 cm².

@Heder Oliveira Dias One the first lin you said the complement of 130 is 30 change 130 to 150

Alex Thang - 6 years, 11 months ago

Ok... thanks...

Heder Oliveira Dias - 6 years, 11 months ago

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