What is the value of (2cos 20°+2cos70°)?

1 Answer
sankarankalyanam
Jan 13, 2018

#color(blue)(2.5634)#

Explanation:

#2cos 20 + 2 cos 70 = 2(cos 20 + cos 70)#

It’s in the form #cos c + cos d and = 2 cos ((c + d)/2)cos ((c-d)/2)#

Hence, # 2 ( cos 20 + cos 70) = 2*2*((cos 20+70)/2)*(cos(20-70)/2)#

#= 4 cos 45 cos (-25) = 4 (1/sqrt 2) (cos 25) = color(blue)(2.5634)#

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