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Home  >>  CBSE XI  >>  Math  >>  Trigonometric Functions
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Evaluate :$\sin(n\pi+(-1)^n\large\frac{\pi}{4})$ where $n$ is an integer,

$\begin{array}{1 1}(A)\;\large\frac{1}{\sqrt 2}&(B)\;\large\frac{1}{2}\\(C)\;1&(D)\;0\end{array} $

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1 Answer

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Toolbox:
  • $\sin (\pi+\theta)=-\sin \theta$
  • $\sin(n\pi+\theta)=(-1)^n\sin \theta$
  • $\sin(-\theta)=-\sin \theta$
$\sin(n\pi+(-1)^n\large\frac{\pi}{4})$
$\Rightarrow (-1)^n\sin[(-1)^n\large\frac{\pi}{4}]$
$\sin(n\pi+\theta)=(-1)^n\sin \theta$
$\Rightarrow (-1)^n(-1)^n\sin \large\frac{\pi}{4}$
$\sin(-\theta)=-\sin \theta$
$\Rightarrow (-1)^{2n}\sin \large\frac{\pi}{4}$
$\Rightarrow \sin \large\frac{\pi}{4}$
$2n$ is even
$\Rightarrow \large\frac{1}{\sqrt 2}$
Hence (A) is the correct answer.
answered May 21, 2014 by sreemathi.v
 

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