Question Details

Let α = 1 sin 60 ° sin 61 ° + 1 sin 62 ° sin 63 ° + …… + 1 sin 118 ° sin 119 °

Then the value of   ( cosec 1 ° α ) 2   is ________

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Correct Answer :

3

Solution :

The correct answer is 3.

Let's analyze the given expression for α:
α=1sin60°sin61°+1sin62°sin63°++1sin118°sin119°

Notice that this sum is not a standard telescoping series because the terms are skipped: (60°,61°), then (62°,63°), and so on, up to (118°,119°).
Let us write the general term as:
Tk=1sin(2k)°sin(2k+1)°
for k=30,31,,59.

To simplify each term, we can multiply and divide by sin1°:
Tk=1sin1°·sin1°sin(2k)°sin(2k+1)°
Since 1°=(2k+1)°-(2k)°, we have:
Tk=1sin1°·sin((2k+1)°-(2k)°)sin(2k)°sin(2k+1)°

Using the identity sin(A-B)=sinAcosB-cosAsinB:
Tk=1sin1°·cot(2k)°-cot(2k+1)°

Thus, we can write α as:
α=cosec1°k=3059cot(2k)°-cot(2k+1)°

This sum expands to:
S=(cot60°-cot61°)+(cot62°-cot63°)++(cot118°-cot119°)

Since cot(180°-θ)=-cotθ, we can group the terms from both ends.
Specifically, let us pair the terms:
- The last negative term is -cot119°=-(-cot61°)=cot61° which cancels out with the first negative term -cot61°.
- The last positive term is cot118°=-cot62° which cancels out with the second positive term cot62°.
In general, for each term in the sum:
cot(2k)°-cot(2k+1)°
Let 2k'=178-2k. The corresponding paired term from the end is:
cot(178-2k)°-cot(179-2k)°=-cot(2k+2)°+cot(2k+1)°

When we add these up, all terms of the form cotθ for θ=61°,62°,,119° cancel in pairs because:
-cot(2k+1)° cancels with -cot(179-2k)°=cot(2k+1)°.
cot(2k)° cancels with cot(180-2k)°=-cot(2k)° for all 2k60° (since 180-60°=120°, which is not in the set of indices, the term cot60° remains unpaired and does not cancel).

Thus, the only term that does not cancel is the very first term:
S=cot60°

We know that:
cot60°=13

Substituting this back into the expression for α:
α=cosec1°·13

We want to find the value of:
cosec1°α2=cosec1°cosec1°·132=32=3

Thus, the final value is 3.

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