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Trigonometric Ratio and Identites question

2019 · 10 Jan · Shift 2 · Q38
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  5. /2019 · 10 Jan · Shift 2 · Q38

Trigonometric Ratio and Identites question

2019 · 10 Jan · Shift 2 · Q38

JEE MainMathematicsTrigonometric Ratio and IdentitesMCQ+4 / −1
The value of cos⁡π22.cos⁡π23 .....cos⁡π210.sin⁡π210\cos {\pi \over {{2^2}}}.\cos {\pi \over {{2^3}}}\,.....\cos {\pi \over {{2^{10}}}}.\sin {\pi \over {{2^{10}}}}cos22π​.cos23π​.....cos210π​.sin210π​ is -
  1. A
    1256{1 \over {256}}2561​
  2. B
    12{1 \over {2}}21​
  3. C
    11024{1 \over {1024}}10241​
  4. D
    1512{1 \over {512}}5121​
View written solutionFree

Correct answer: D

  1. We need to evaluate

P=cos⁡π22⋅cos⁡π23⋯cos⁡π210⋅sin⁡π210.P=\cos\frac{\pi}{2^2}\cdot \cos\frac{\pi}{2^3}\cdots \cos\frac{\pi}{2^{10}}\cdot \sin\frac{\pi}{2^{10}}.P=cos22π​⋅cos23π​⋯cos210π​⋅sin210π​.

That is,

P=(∏k=210cos⁡π2k)sin⁡π210.P=\left(\prod_{k=2}^{10}\cos\frac{\pi}{2^k}\right)\sin\frac{\pi}{2^{10}}.P=(∏k=210​cos2kπ​)sin210π​.

  1. Use the standard identity

sin⁡2x=2sin⁡xcos⁡x\sin 2x = 2\sin x\cos xsin2x=2sinxcosx

which gives

sin⁡x=2sin⁡x2cos⁡x2.\sin x = 2\sin\frac{x}{2}\cos\frac{x}{2}.sinx=2sin2x​cos2x​.

Applying repeatedly,

sin⁡x=2nsin⁡x2ncos⁡x2cos⁡x22⋯cos⁡x2n.\sin x = 2^n \sin\frac{x}{2^n}\cos\frac{x}{2}\cos\frac{x}{2^2}\cdots \cos\frac{x}{2^n}.sinx=2nsin2nx​cos2x​cos22x​⋯cos2nx​.

Hence,

cos⁡x2cos⁡x22⋯cos⁡x2nsin⁡x2n=sin⁡x2n.\cos\frac{x}{2}\cos\frac{x}{2^2}\cdots \cos\frac{x}{2^n}\sin\frac{x}{2^n} = \frac{\sin x}{2^n}.cos2x​cos22x​⋯cos2nx​sin2nx​=2nsinx​.

  1. Here, take x=π2x=\dfrac{\pi}{2}x=2π​. Then

cos⁡π22cos⁡π23⋯cos⁡π210sin⁡π210=sin⁡(π/2)29.\cos\frac{\pi}{2^2}\cos\frac{\pi}{2^3}\cdots \cos\frac{\pi}{2^{10}}\sin\frac{\pi}{2^{10}} = \frac{\sin(\pi/2)}{2^9}.cos22π​cos23π​⋯cos210π​sin210π​=29sin(π/2)​.

Why 292^929? Because the cosine factors run from 222^222 to 2102^{10}210, i.e. there are 999 cosine terms.

  1. Now,

sin⁡π2=1.\sin\frac{\pi}{2}=1.sin2π​=1.

So,

P=129=1512.P=\frac{1}{2^9}=\frac{1}{512}.P=291​=5121​.

  1. Therefore the correct option is

1512.\boxed{\frac{1}{512}}.5121​​.

So, option D\boxed{D}D​ is correct.

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