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Question

1+cos 56+cos 58cos 66=

 



Solution

The correct option is C


1+cos 56+cos 58cos 66=
=2cos228+2sin 62.sin 4
=2cos228+2cos 28.sin 4
=2cos28(cos 28+cos 86)
=2cos28.2cos 57.cos 29
=4cos 28cos 29sin 33
Aliter: Apply the condition identify

cos A+cos Bcos C=1+4cosA2cosB2sinc2
[56+58+66=180]
We get the value of required expression equal to 4cos28 cos29 sin33.

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