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Grade: 11

                        

All angle are in degrees Cos20+cos40+cos60-4cos10.cos20.cos30 is equal to??????

2 years ago

Answers : (1)

Aditya Gupta
2065 Points
							
Cos20+cos40=2cos30cos10
4cos10.cos20=2cos30+2cos10
so, 4cos10.cos20cos30=2[cos30+cos10]cos30
so, subtracting
Cos20+cos40 – 4cos10.cos20cos30=2cos30[cos10 – (cos30+cos10)]= –2cos^230= –3/2
adding cos60 to both sides
Cos20+cos40 – 4cos10.cos20cos30+cos60= –3/2+1/2 =  – 1
2 years ago
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