1.

Express the following as a sum or difference of two trigonometric functions: 2cos 35° cos 75°

Answer»

2cos 35° cos75° 

= cos(35° + 75°) + cos (35° – 75°) 

= cos 110° + cos (-40)° 

= cos 110° + cos 40° … [∵ cos(-θ) = cos θ]



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