0.96

Geometry Level 2

If 0 < θ < π 2 0 < \theta < \frac{\pi}{2} and sin θ = 24 25 , \sin \theta=\frac{24}{25}, what is the value of

cos θ 2 sin θ 2 + cos θ 2 ? \frac{\cos \frac{\theta}{2}}{\sin \frac{\theta}{2}+\cos \frac{\theta}{2}}?

2 7 \frac{2}{7} 3 7 \frac{3}{7} 4 7 \frac{4}{7} 5 7 \frac{5}{7}

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5 solutions

Chew-Seong Cheong
May 16, 2015

It is given that: sin θ = 24 25 \space \sin{\theta} = \frac{24}{25}

cos θ = 1 ( 24 25 ) 2 = 2 5 2 2 4 2 2 5 2 = ( 25 24 ) ( 25 + 24 ) 2 5 2 = 49 2 5 2 = 7 25 \Rightarrow \cos{\theta} = \sqrt{1- \left( \frac{24}{25} \right)^2} = \sqrt{ \frac{25^2 - 24^2}{25^2}} = \sqrt{ \frac{(25 - 24)(25+24)}{25^2}} = \sqrt{ \frac{49}{25^2}} = \frac{7}{25}

cos θ 2 sin θ 2 + cos θ 2 = cos θ 2 sin θ 2 + cos θ 2 × 2 cos θ 2 2 cos θ 2 = 2 cos 2 θ 2 2 sin θ 2 cos θ 2 + 2 cos 2 θ 2 = 2 cos 2 θ 2 1 + 1 2 sin θ 2 cos θ 2 + 2 cos 2 θ 2 1 + 1 = cos θ + 1 sin θ + cos θ + 1 = 7 25 24 25 + 7 25 + 1 = 32 56 = 4 7 \begin{aligned} \frac {\cos{\frac{\theta}{2}}}{\sin{\frac{\theta}{2}}+\cos{\frac{\theta}{2}}} & = \frac {\cos{\frac{\theta}{2}}}{\sin{\frac{\theta}{2}}+\cos{\frac{\theta}{2}}} \times \frac{2\cos{\frac{\theta}{2}}}{2\cos{\frac{\theta}{2}}} \\ & = \frac {2\cos^2{\frac{\theta}{2}}}{2\sin{\frac{\theta}{2}}\cos{\frac{\theta}{2}}+2\cos^2{\frac{\theta}{2}}} \\ & = \frac {2\cos^2{\frac{\theta}{2}}-1+1}{2\sin{\frac{\theta}{2}}\cos{\frac{\theta}{2}}+2\cos^2{\frac{\theta}{2}}-1+1} \\ & = \frac {\cos{\theta}+1}{\sin{\theta}+\cos{\theta}+1} = \frac {\frac{7}{25}}{\frac{24}{25}+\frac{7}{25}+1} = \frac{32}{56} = \boxed{\frac{4}{7}}\end{aligned}

Ahmed Hessin
May 16, 2015

Pranav Pant
May 16, 2015

Dhanya Jose
May 22, 2015

sin(theta)=24/25 so (theta)=sin inverse of 24/25 which is 73.74 nw substitute in place of theta as 73.74

Let x= cos (theta), y= sin(theta). Let alpha= cos(theta/2), beta=sin(theta/2).

Let the value of the given expression be V. Then.

V=alpha/(alpha+beta)

=alpha(alpha-beta)/(alpha^2-beta^2)

= (2alpha^2-2alpha beta)/[2(alpha^2-beta^2)]

From the definitions of alpha and beta,

2 alpha beta= sin(theta)=y

2 alpha^2-1=alpha^2-beta^2=cos(theta)=x, i.e., 2 alpha^2=1+x

Therefore,

V=(1+x-y)/(2x)

y=24/25 implies x= 7/25 (since x^2+y^2=1)

Therefore, V= (25+7-24)/14=8/14=4/7

It is easier if you multiply it by 2\sin(\frac{theta}{2}). Farzad Saeidi - now

Farzad Saeidi - 2 years, 5 months ago

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