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Geometry Level 3

If tan θ = 3 4 \tan{\theta} = \dfrac{3}{4} and 0 < θ < π 2 0 < \theta < \dfrac{\pi}{2} , let sin θ \sin{\theta} and cos θ \cos{\theta} be two sides of a triangle with the third side having a length of 1 1 . The area of this triangle is of the form a b \dfrac{a}{b} , where a a and b b are coprime positive integers. Find a + b a+b .


The answer is 31.

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1 solution

Akeel Howell
Feb 17, 2017

Given the dimensions of this triangle, we can consider the unit circle with a radius of 1 1 and points the circumference with coordinate values of ( cos θ , sin θ ) (\cos{\theta}, \sin{\theta}) . Here, we are clearly dealing with a right triangle with a hypotenuse of 1 1 .

When tan θ = 3 4 \tan{\theta} = \dfrac{3}{4} for 0 < θ < π 2 0<\theta<\dfrac{\pi}{2} , sin θ = 3 3 2 + 4 2 = 3 5 \sin{\theta} = \dfrac{3}{\sqrt{3^2+4^2}} = \dfrac{3}{5} and cos θ = 4 3 2 + 4 2 = 4 5 \cos{\theta} = \dfrac{4}{\sqrt{3^2+4^2}} = \dfrac{4}{5} . Thus, the area of such a triangle is 1 2 3 5 4 5 = 6 25 \dfrac{1}{2} \cdot \dfrac{3}{5} \cdot \dfrac{4}{5} = \dfrac{6}{25}

a + b = 6 + 25 = 31. \therefore a+b = 6+25 = \boxed{31.}

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