Please explain the marked part of the sum (trigonometry ---> sums and products) given in the attachment. Please answer this fast. Tomorrow is exam. Please don't spam it.
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Step-by-step explanation:
In first marked step,
1/2 is taken as common, so 2 is multiplied to cos20°cos80° to balance it.
In second marked step,
2cos20°cos80°=cos(80°+20°) + cos(80°-20°) = cos 100° + cos60°
...[as you have done it before in case of 2cos20°cos40°= cos60°+cos20°]
In third marked step,
There is a mistake.
cos80° = sin10°
cos100° =cos(90°+10°) = - sin10°
cos60° = 1/2
Thus, the third step must be...
1/4*{sin10°-sin10°+1/2} = 1/4*{1/2} = 1/8
GLAD TO HELP YOU.
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