Question

cos20° + cos40° + cos60°-
4 cos 10 cos 20 cos 30​

Answer 1

Step-by-step explanation:

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