If 0 < θ < 2 π and sin θ = 2 5 2 4 , what is the value of
sin 2 θ + cos 2 θ cos 2 θ ?
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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
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It is given that: sin θ = 2 5 2 4
⇒ cos θ = 1 − ( 2 5 2 4 ) 2 = 2 5 2 2 5 2 − 2 4 2 = 2 5 2 ( 2 5 − 2 4 ) ( 2 5 + 2 4 ) = 2 5 2 4 9 = 2 5 7
sin 2 θ + cos 2 θ cos 2 θ = sin 2 θ + cos 2 θ cos 2 θ × 2 cos 2 θ 2 cos 2 θ = 2 sin 2 θ cos 2 θ + 2 cos 2 2 θ 2 cos 2 2 θ = 2 sin 2 θ cos 2 θ + 2 cos 2 2 θ − 1 + 1 2 cos 2 2 θ − 1 + 1 = sin θ + cos θ + 1 cos θ + 1 = 2 5 2 4 + 2 5 7 + 1 2 5 7 = 5 6 3 2 = 7 4