1+ cos 56 = 2 cos^2 28 as cos 2t = 2 cos^2 t - 1
cos 58 = 2 cos^ 29 -1
cos 66 = 2cos ^2 33 -1
so 1+ cos 56 + cos 58 - cos 66
= 2 cos^2 28 + 2 cos^2 29 - 2 cos^2 33
= 2 ( cos^2 28 + cos ^2 29 - cos ^2 33)
now 28+29+33 = 90
if we prove cos ^2 A + cos^2 B - cos^2 c = 2 cos A cos B sin C when A+B+C = pi.2
then we are through
2 comments:
How to solve the problem if
"2008/009) Prove; cos 56 + cos 58 - cos 66 = 4*cos 28. cos 29. sin 33 = - 1
There is a typo in the above problem.
Prove; cos 56 + cos 58 - cos 66 = 4*cos 28. cos 29. sin 33 - 1
is same as I have mentioned by taking 1 to the right
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