All angle are in degrees Cos20+cos40+cos60-4cos10.cos20.cos30 is equal to??????
All angle are in degrees
Cos20+cos40+cos60-4cos10.cos20.cos30 is equal to??????
harsh jain , 6 Years ago
Grade 11
Trigonometry> All angle are in degrees Cos20+cos40+cos6...
1 Answers
Aditya Gupta
Last Activity: 6 Years ago
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
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