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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

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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

## #3 2009-06-10 15:36:35

nurshodiq
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### Re: trigonometri problem

Last edited by nurshodiq (2009-06-10 15:44:24)

## #4 2009-06-11 00:27:38

Kurre
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### Re: trigonometri problem

Last edited by Kurre (2009-06-11 02:01:21)

## #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)