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In triangle ABC, cos A = 2/3 and cos B = 2/9. Given the perimeter of the triangle ABC is 24, compute the area of triangle ABC.

 Aug 3, 2022

Best Answer 

 #1
avatar+113 
+2

Find angle A = cos-1(2/3) = 48.19 (Deg), and angle B = cos-1(2/9) = 77.16 (Deg), so angle C = 180 - 48.19 - 77.16 = 54.65 (Deg).

 

The Sine Rule states that a/sin(A) = b/sin(B) = c/sin(C). We also know that a + b + c = 24.

Let's try and use both of these to express a and b by means of c.
a = c x (sin(A)) / sin(C) = c * 0.745 / 0.816 = 0.913 x c, b = c x (sin(B)) / sin(C) = c x 0.975 / 0.816 = 1.195 x c.

 

Substituting (exchanging) a and b for these gives us that: a + b + c = 24 is the same as 0.913 c + 1.195 c + c = 24,
or 3.108 c = 24. Divding on both sides gives us c = 24 / 3.108 = 7.722.

 

Now we can back-substitute and find a and b, a = 0.913c = 0.913 x 7.722 = 7.050, and b = 1.195c = 1.195 x 7.722 = 9.228.

 

We can find the area of a trangle given all it's sides by using Heron's formula:
(Se: https://www.mathsisfun.com/geometry/herons-formula.html )

We find s = 1/2 x ( a + b + c ) = 1/2 (7.050 + 9.228 + 7.722) = 1/2 (24) = 12.
[This being a whole number indicates we are on the right path)
 

The area is given by \(A =\sqrt{s(s-a)(s-b)(s-c)}\) , so \(A =\sqrt{12\times4.59\times2.772\times4.278} = \sqrt{704.402} \approx 26.54\)
 

heron's formula

Hope this helps!

 Aug 3, 2022
 #1
avatar+113 
+2
Best Answer

Find angle A = cos-1(2/3) = 48.19 (Deg), and angle B = cos-1(2/9) = 77.16 (Deg), so angle C = 180 - 48.19 - 77.16 = 54.65 (Deg).

 

The Sine Rule states that a/sin(A) = b/sin(B) = c/sin(C). We also know that a + b + c = 24.

Let's try and use both of these to express a and b by means of c.
a = c x (sin(A)) / sin(C) = c * 0.745 / 0.816 = 0.913 x c, b = c x (sin(B)) / sin(C) = c x 0.975 / 0.816 = 1.195 x c.

 

Substituting (exchanging) a and b for these gives us that: a + b + c = 24 is the same as 0.913 c + 1.195 c + c = 24,
or 3.108 c = 24. Divding on both sides gives us c = 24 / 3.108 = 7.722.

 

Now we can back-substitute and find a and b, a = 0.913c = 0.913 x 7.722 = 7.050, and b = 1.195c = 1.195 x 7.722 = 9.228.

 

We can find the area of a trangle given all it's sides by using Heron's formula:
(Se: https://www.mathsisfun.com/geometry/herons-formula.html )

We find s = 1/2 x ( a + b + c ) = 1/2 (7.050 + 9.228 + 7.722) = 1/2 (24) = 12.
[This being a whole number indicates we are on the right path)
 

The area is given by \(A =\sqrt{s(s-a)(s-b)(s-c)}\) , so \(A =\sqrt{12\times4.59\times2.772\times4.278} = \sqrt{704.402} \approx 26.54\)
 

heron's formula

Hope this helps!

tuffla2022 Aug 3, 2022
 #2
avatar+124596 
0

Very nice, Tuffla  !!!

 

 

cool cool cool

CPhill  Aug 3, 2022

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