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Algebra Level 2

True or False :

Z = ι sin ( ι π 6 ) is a real number \Large Z = \iota \sin\left(\iota \dfrac{\pi}{6}\right) \ \text{is a real number}

Details :

  • ι = 1 \iota = \sqrt{-1}
True Can't say False

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2 solutions

Akhil Bansal
Oct 18, 2015

From Euler's Formula , we know that
sin θ = e ι θ e ι θ 2 ι \large \sin\theta = \dfrac{ e^{\iota\theta} - e^{{-\iota}{\theta}}}{2\iota}

Z = ι sin ( ι π 6 ) Z = \iota \sin\left(\iota \dfrac{\pi}{6}\right)

Z = e ι ( ι π 6 ) e ι ( ι π 6 ) 2 Z = \dfrac{ e^{\iota\left( \iota\frac{\pi}{6}\right)} - e^{-\iota\left( -\iota\frac{\pi}{6}\right)}}{2}

Z = e π 6 e π 6 2 = 0.548 (real number) Z = \dfrac{ e^{- \frac{\pi}{6}} - e^{\frac{\pi}{6}}}{2} = -0.548 \ \text{(real number)}

Akash Shukla
Jun 12, 2016

As we know, the series of sin(x) contains odd powers of x x , sin x = x x 3 3 ! + x 5 5 ! x 7 7 ! + \begin{aligned} \sin x &=& x - \dfrac{x^3}{3!} + \dfrac{x^5}{5!} - \dfrac{x^7}{7!} + \cdots \end{aligned} ,

So here if we put x = A i x=Ai , A R A\in R then the series will have complex terms + i +i or i -i and on multiplying with i i , the series will have + 1 +1 or 1 -1 in place of i i everywhere, so it will be completely a real number

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