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Prove that cos20cos40cos60cos80=1/16 |
| Answer» cos20 cos40 1/2 cos80 (cos60 = 1/2)multiply and divide by 21/4 (2 cos20 cos40 cos80)1/4 (cos(20+80)+ cos(20-80)) cos40 (2cosa cosb= cos(a+b) + cos(a-b))1/4 (cos(-60)+ cos(100)) cos401/4(1/2 + cos100)cos401/8 cos40+ 1/4 (cos40 cos100)multiplt nd divide by 22/2(1/8 cos40) + 1/8(2 cos40 cos100)1/8 cos40+ 1/8 (cos140+ cos(-60)) (2cosa cosb= cos(a+b) cos(a-b))1/8 cos40+ 1/8 cos140 + 1/16 (cos60= 1/2)1/8(cos40+cos140) + 1/161/8(2 cos90 cos(-50)) + 1/16cos90= 01/16= r.h.shence proved | |