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Prove the following:
cos(π+θ)cos(θ)sin(πθ)cos(π2+θ)=cot2θ

Answer
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Hint: We will be using the concepts of trigonometric function to solve the problem. We will be using the trigonometric formulae like:
cos(π+θ)=cosθcos(θ)=cosθsin(πθ)=sinθcos(π2+θ)=sinθcosθsinθ=cotθ

Complete step-by-step answer:
Now, we have to prove that,
cos(π+θ)cos(θ)sin(πθ)cos(π2+θ)=cot2θ
Now, we know that for trigonometric ratios of supplementary angles are determined as,
seo images

The figure shows the quadrant in which the trigonometric ratios are positive.
Now, it is important to note that the trigonometric ratio of an angle and its supplement is the same in magnitude but the sign depends on the quadrant in which it lies. For example: sin(180θ)=sin(θ) as sin is positive in the second quadrant. Now, the same goes for complementary angles but in those the co-functions are the same and the sign depends on the quadrant in which the trigonometric function lies. For example: sin(90θ)=cosθ.
Now, we take LHS,
cos(π+θ)cos(θ)sin(πθ)cos(π2+θ)
Now, we have,
cos(π+θ)=cosθcos(θ)=cosθsin(πθ)=sinθcos(π2+θ)=sinθ
So, the LHS become,
cosθ×cosθsinθ×(sinθ)=cos2θsin2θ
Now, we know that cosθsinθ=cotθ.
Therefore, we have,
cos(π+θ)cos(θ)sin(πθ)cos(π2+θ)=cot2θ
Now, since LHS = RHS
Hence proved.

Note: To solve these type of questions it is important to note that we have used the ASTC quadrant system in which all the trigonometric ratios are positive in the first quadrant whereas only sine, cosec is positive in the second quadrant and tan, cot is positive in the third quadrant and cos, sec is positive in the fourth quadrant like
seo images

also it is important to remember the trigonometric formula like:
cos(π+θ)=cosθsin(π2+θ)=cos(θ)cos(π2+θ)=cosθ
Hence, remembering these formulae will help to shorten the solution.
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