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Trigonometric Ratio and Identites question

2022 · 25 Jul · Shift 2 · Q36
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  5. /2022 · 25 Jul · Shift 2 · Q36

Trigonometric Ratio and Identites question

2022 · 25 Jul · Shift 2 · Q36

JEE MainMathematicsTrigonometric Ratio and IdentitesMCQ+4 / −1
2sin⁡(π22)sin⁡(3π22)sin⁡(5π22)sin⁡(7π22)sin⁡(9π22)2 \sin \left(\frac{\pi}{22}\right) \sin \left(\frac{3 \pi}{22}\right) \sin \left(\frac{5 \pi}{22}\right) \sin \left(\frac{7 \pi}{22}\right) \sin \left(\frac{9 \pi}{22}\right)2sin(22π​)sin(223π​)sin(225π​)sin(227π​)sin(229π​) is equal to :
  1. A
    316\frac{3}{16}163​
  2. B
    116\frac{1}{16}161​
  3. C
    132\frac{1}{32}321​
  4. D
    932\frac{9}{32}329​
View written solutionFree

Correct answer: B

  1. Let P=2sin⁡(π22)sin⁡(3π22)sin⁡(5π22)sin⁡(7π22)sin⁡(9π22).P=2\sin\left(\frac{\pi}{22}\right)\sin\left(\frac{3\pi}{22}\right)\sin\left(\frac{5\pi}{22}\right)\sin\left(\frac{7\pi}{22}\right)\sin\left(\frac{9\pi}{22}\right).P=2sin(22π​)sin(223π​)sin(225π​)sin(227π​)sin(229π​).

We need to evaluate this product.

  1. Use the standard identity ∏k=1n−1sin⁡(kπn)=n2n−1.\prod_{k=1}^{n-1}\sin\left(\frac{k\pi}{n}\right)=\frac{n}{2^{n-1}}.∏k=1n−1​sin(nkπ​)=2n−1n​.

For n=11n=11n=11, ∏k=110sin⁡(kπ11)=11210.\prod_{k=1}^{10}\sin\left(\frac{k\pi}{11}\right)=\frac{11}{2^{10}}.∏k=110​sin(11kπ​)=21011​.

  1. Now use symmetry: sin⁡(kπ11)=sin⁡(π−kπ11)=sin⁡((11−k)π11).\sin\left(\frac{k\pi}{11}\right)=\sin\left(\pi-\frac{k\pi}{11}\right)=\sin\left(\frac{(11-k)\pi}{11}\right).sin(11kπ​)=sin(π−11kπ​)=sin(11(11−k)π​).

Hence, ∏k=110sin⁡(kπ11)=[sin⁡(π11)sin⁡(2π11)sin⁡(3π11)sin⁡(4π11)sin⁡(5π11)]2.\prod_{k=1}^{10}\sin\left(\frac{k\pi}{11}\right)=\left[\sin\left(\frac{\pi}{11}\right)\sin\left(\frac{2\pi}{11}\right)\sin\left(\frac{3\pi}{11}\right)\sin\left(\frac{4\pi}{11}\right)\sin\left(\frac{5\pi}{11}\right)\right]^2.∏k=110​sin(11kπ​)=[sin(11π​)sin(112π​)sin(113π​)sin(114π​)sin(115π​)]2.

Therefore, sin⁡(π11)sin⁡(2π11)sin⁡(3π11)sin⁡(4π11)sin⁡(5π11)=1125.\sin\left(\frac{\pi}{11}\right)\sin\left(\frac{2\pi}{11}\right)\sin\left(\frac{3\pi}{11}\right)\sin\left(\frac{4\pi}{11}\right)\sin\left(\frac{5\pi}{11}\right)=\frac{\sqrt{11}}{2^5}.sin(11π​)sin(112π​)sin(113π​)sin(114π​)sin(115π​)=2511​​.

  1. Our given angles are π22,3π22,5π22,7π22,9π22,\frac{\pi}{22},\frac{3\pi}{22},\frac{5\pi}{22},\frac{7\pi}{22},\frac{9\pi}{22},22π​,223π​,225π​,227π​,229π​, which are exactly (2r−1)π22,r=1,2,3,4,5.\frac{(2r-1)\pi}{22},\quad r=1,2,3,4,5.22(2r−1)π​,r=1,2,3,4,5.

So we use the identity for product of sines of odd multiples: ∏r=1msin⁡((2r−1)π4m+2)=12m.\prod_{r=1}^{m}\sin\left(\frac{(2r-1)\pi}{4m+2}\right)=\frac{1}{2^m}.∏r=1m​sin(4m+2(2r−1)π​)=2m1​.

Here 4m+2=224m+2=224m+2=22, so m=5m=5m=5. Thus \sin\left(\frac{\pi}{22}\right)\sin\left(\frac{3\pi}{22}\right)\sin\left(\frac{5\pi}{22}\right)\sin\left(\frac{7\pi}{22}\right)\sin\left(\frac{9\pi}{22}\right)=\frac{1}{2^5}= rac{1}{32}.

Hence P=2⋅132=116.P=2\cdot \frac{1}{32}=\frac{1}{16}.P=2⋅321​=161​.

  1. Therefore the correct option is 116\boxed{\frac{1}{16}}161​​ which is option B\boxed{\text{B}}B​.

  2. Verification with stored answer:

  • Derived answer: B\boxed{\text{B}}B​
  • Stored correct answer: B\boxed{\text{B}}B​

They agree.

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