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Evaluate Trigonometric Series Sum and Cosecant Ratio Squared

Let

α=1sin60sin61+1sin62sin63++1sin118sin119.\alpha = \frac{1}{\sin 60^\circ \sin 61^\circ} + \frac{1}{\sin 62^\circ \sin 63^\circ} + \dots + \frac{1}{\sin 118^\circ \sin 119^\circ}.

Then the value of

(csc1α)2\left( \frac{\csc 1^\circ}{\alpha} \right)^2

is ______.

Official Numerical Answer3

Step-by-Step Solution

To evaluate the given sum, we multiply both sides of the expression for α\alpha by sin1\sin 1^\circ:

sin1α=sin(6160)sin60sin61+sin(6362)sin62sin63++sin(119118)sin118sin119\sin 1^\circ \alpha = \frac{\sin(61^\circ - 60^\circ)}{\sin 60^\circ \sin 61^\circ} + \frac{\sin(63^\circ - 62^\circ)}{\sin 62^\circ \sin 63^\circ} + \dots + \frac{\sin(119^\circ - 118^\circ)}{\sin 118^\circ \sin 119^\circ}

Using the identity sin(yx)sinxsiny=cotxcoty\frac{\sin(y-x)}{\sin x \sin y} = \cot x - \cot y, we rewrite the series as:

sin1α=(cot60cot61)+(cot62cot63)++(cot118cot119)\sin 1^\circ \alpha = (\cot 60^\circ - \cot 61^\circ) + (\cot 62^\circ - \cot 63^\circ) + \dots + (\cot 118^\circ - \cot 119^\circ)

Grouping the positive and negative terms separately:

sin1α=(k=3059cot(2k))(k=3059cot((2k+1)))\sin 1^\circ \alpha = \left( \sum_{k=30}^{59} \cot(2k^\circ) \right) - \left( \sum_{k=30}^{59} \cot((2k+1)^\circ) \right)

By symmetric cancellation using cot(180θ)=cotθ\cot(180^\circ - \theta) = -\cot \theta, all pairs in the second sum cancel to zero, and in the first sum all terms cancel except for cot60\cot 60^\circ:

sin1α=cot60=13\sin 1^\circ \alpha = \cot 60^\circ = \frac{1}{\sqrt{3}}

Rearranging gives:

csc1α=3\frac{\csc 1^\circ}{\alpha} = \sqrt{3}

Squaring both sides yields:

(csc1α)2=3\left( \frac{\csc 1^\circ}{\alpha} \right)^2 = 3

Evaluate Trigonometric Series Sum and Cosecant Ratio Squared | Mathematics PYQ Solution - JEE Challenger