Question

(xii) 4(cos^3 10° + sin^3 20°) = 3(cos 10° + sin 2°
)


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Answer 1

Answer:

Step-by-step explanation:

cos 3θ = 4cos3 θ - 3cosθ

4cos3 θ = cos3θ + 3 cosθ

Taking θ = 10°

4cos3 10° = cos30° + 3cos10° ... ( 1 )

sin3θ = 3sinθ - 4sin3 θ

4sin3 θ = 3sinθ - sin3θ

Taking θ = 20°

4sin3 20° = 3sin20° - sin60° ... ( 2 )

Adding ( 1 ) and ( 2 ) Equation.

4(cos3 10° + sin3 20°) = 3(cos10° + sin20°) + cos30° - sin60°

4(cos3 10° + sin3 20°) = 3(cos10° + sin20°)                                                             [as we know that cos30=sin60]