If a line makes an angles α,β,γ with positive axes. Then the range of sinαsinβ+sinβsinγ+sinαsinγ is:
A
[−12,1]
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B
[12,2]
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C
[−1,2]
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D
(−1,2]
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Solution
The correct option is C[−1,2] We know, sin2α+sin2β+sin2γ=2
and (sinα+sinβ+sinγ)2≥0 ⇒2+2(Σsinαsinβ)≥0 ⇒Σsinαsinβ≥−1.....(1)
Also, Σ(sinα−sinβ)2≥0⇒2(sin2α+sin2β+sin2γ)−2Σsinαsinβ≥0⇒4−2Σsinαsinβ≥0 ⇒2Σsinαsinβ≤4⇒Σsinαsinβ≤2.....(2)
Range =[−1,2]