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#1 2009-06-05 10:15:42
- nurshodiq
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trigonometri problem
1. find value of
2. find value of
3. if , prove
4. if , prove
Last edited by nurshodiq (2009-06-05 11:20:41)
#2 2009-06-08 13:08:47
- baskar
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Re: trigonometri problem
answer for (1) find the value of cos^2 (18) + cos^2 (54):
cos^2 (18) = [10+2 sqrt(5)]/16 = 0.9045085
we know cos3x = 4cos^3 (x) - 3cos(x)
=> cos(54) = 4 cos^3 (18) - 3cos(18) = 4 (0.74001063) - 3 (0.9045085)
=> cos(54) = 2.96004252 - 2.71352550 = 0.24651702
=> cos^2 (54) = (0.24651702)^2 = 0.06077
=> cos^2 (18) + cos^2 (54) = 0.9045085 + 0.0607700 = 0.9652785
cheers, baskar
#3 2009-06-10 15:36:35
- nurshodiq
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Re: trigonometri problem
thanks baskar, i had answer for #1. but different with yours,
Last edited by nurshodiq (2009-06-10 15:44:24)
#5 2009-06-11 02:15:29
- Kurre
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Re: trigonometri problem
I wanted to find a solution that uses the result from problem 3, which took a while and is probably not the easiest one, but here it is anyway:
Last edited by Kurre (2009-06-11 02:19:38)
#6 2009-06-11 22:49:25
- nurshodiq
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Re: trigonometri problem
i have a way to answer #4,
if
and
, so we have
multiple both sides by 4 we have
thanks
Last edited by nurshodiq (2009-06-11 23:25:02)