AravindG
  • AravindG
how to find value of sin 18
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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jamiebookeater
  • jamiebookeater
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anonymous
  • anonymous
Take a calculator, and hit the "sin" button and type 18 in. You (may) need a scientific/graphing/google calculator. If you are asking how to do so by hand, I am not qualified to tell you.
AravindG
  • AravindG
there must be a method using the formulas
AravindG
  • AravindG
turing??

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TuringTest
  • TuringTest
I am thinking....
anonymous
  • anonymous
(18*5)=90
AravindG
  • AravindG
so?
TuringTest
  • TuringTest
all I got is the fact that we can write this as\[\sin(\frac\pi{10})\]thinking if we can go somewhere from there...
TuringTest
  • TuringTest
looking at some way to write this such that we can apply an identity to it...
anonymous
  • anonymous
http://answers.yahoo.com/question/index?qid=20061025091609AASJNUn
AravindG
  • AravindG
whoa!!
TuringTest
  • TuringTest
why is \[\sin(3x)=\cos(2x)\]?
anonymous
  • anonymous
Most of peoples are not smart....they are clever
campbell_st
  • campbell_st
use the power series expansion \[\sin(x) = x/1! - x^3/3! + x^5/5! -x^7/7!.....\]
AravindG
  • AravindG
hw put valu 18>??
AravindG
  • AravindG
bt it goes on
anonymous
  • anonymous
if x= 18, 2x+3x=90
AravindG
  • AravindG
Another way to do this is to start with a regular pentagon where the length of each side is one unit. I'll outline the steps. (You can fill in the details.) Call the pentagon ABCDE. Draw diagonals AC and BE, which intersect at point F. Using the fact that each angle of the pentagon is 108 degrees, you can prove the triangles BFC and AFE are isosceles. Therefore the lengths of line segments CF and EF are also 1 unit. Also note the angles for the triangles are 72-72-36. Now show that ABF and CAB are similar triangles. Since they are similar, this means BE/AB = AB/BF. Since BE = EF + BF, substitute to get (EF + BF)/AB = AB/BF. Let's define x to be the length of BF. Then, since EF = 1, the equation becomes (1 + x)/1 = 1/x, or x^2 + x - 1 = 0. Use the quadratic equation to get x = (sqrt(5) - 1) / 2. Finally, let's create an 18-72-90 by drawing the altitude of triangle BFC from vertex C to side BF. Since BC = FC, the altitude hits BF at the midpoint of BF, which we will call point G. So, (a) triangle GBE is an 18-72-90 triangle, (b) BG = BF/2 = x/2. Using this 18-72-90 triangle, sin 18 degrees = BG / BC = x/2. Therefore sin 18 = (sqrt(5) - 1) / 4.
AravindG
  • AravindG
i liked this
anonymous
  • anonymous
2x=90-3x
anonymous
  • anonymous
cos both side
AravindG
  • AravindG
k
anonymous
  • anonymous
cos(2x)=sin(3x)
AravindG
  • AravindG
thx evryone
anonymous
  • anonymous
catch it?
TuringTest
  • TuringTest
gotchya, thx
AravindG
  • AravindG
campbell pls tell ur method

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