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Mathematics

Without using trigonometric tables, evaluate the following:

sin 65°cos 25°+cos 32°sin 58°sin 28° sec 62°+cosec2 30°\dfrac{\text{sin 65°}}{\text{cos 25°}} + \dfrac{\text{cos 32°}}{\text{sin 58°}} - \text{sin 28° sec 62°} + \text{cosec}^2 \space 30°

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Answer

We need to find the value of

sin 65°cos 25°+cos 32°sin 58°sin 28° sec 62°+cosec2 30°\dfrac{\text{sin 65°}}{\text{cos 25°}} + \dfrac{\text{cos 32°}}{\text{sin 58°}} - \text{sin 28° sec 62°} + \text{cosec}^2 \space 30°

As sec θ = 1cos θ\dfrac{1}{\text{cos θ}}

The above equation can be written as,

sin 65°cos (90 - 65)°+cos (90 - 58)°sin 58°sin 28°1cos (90 - 28)°+cosec2 30°\dfrac{\text{sin 65°}}{\text{cos (90 - 65)°}} + \dfrac{\text{cos (90 - 58)°}}{\text{sin 58°}} - \text{sin 28°} \dfrac{1}{\text{cos (90 - 28)°}} + \text{cosec}^2 \space 30°

As, cos(90 - θ) = sin θ and cosec 30° = 2. Using in above equation,

=sin 65°sin 65°+sin 58°sin 58°sin 28°1sin 28°+22=1+11+4=61=5.= \dfrac{\text{sin 65°}}{\text{sin 65°}} + \dfrac{\text{sin 58°}}{\text{sin 58°}} - \text{sin 28°} \dfrac{1}{\text{sin 28°}} + 2^2 \\[1em] = 1 + 1 - 1 + 4 \\[1em] = 6 - 1 \\[1em] = 5.

Hence, the value of the expression is 5.

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