Solve the equation sin(x+3)=sinx, 0=<x=<2pi to 4 decimal places. The product of the two solutions is:?

2021-04-07 10:30 am
Need help with this problem pls

回答 (8)

2021-04-07 9:13 pm
sin(x+3)=sin(x)
=>
sin(x+3)-sin(x)=0
=>
2cos(x+3/2)sin(3/2)=0
[sum to product formula]
=>
cos(x+3/2)=0
=>
x=(2k+1)pi/2-3/2
=>
x=[(2k+1)pi-3]/2, where k=0, 1
k=0=>x0=pi/2-3/2
k=1=>x1=(3pi/2-3/2)
=>
x0x1=
3(pi-3)(pi-1)/4=
0.2274253...
2021-04-07 10:56 am
The sine curve has a period of 2*pi (roughly 6.283)
Your question is looking for an angle x which has the same value as it would have 3 radians later.
Doing it "mechanically":
draw a sine curve which measured 6.283 (inches, cm, whatever, as long as you keep the same units throughout). Measure off 3 units on an edge, then run that instrument along the curve, until the values match.
You will find that x must be just after the positive crest (just slightly more than pi/2 radians) putting the (x+3) angle just before the next positive crest

(there will also be another answer where x is just after the negative minimum dip -- making x slightly more than 3pi/2).

---

Using trig identities:

sin(x+3) = sin(x)cos(3) + cos(x)sin(3)
You want this to equal sin(x)

sin(x) = sin(x)cos(3) + cos(x)sin(3)
(remember that cos(3) is "just a number"-- it should not bother you)
(same thing for sin(3))

sin(x) - sin(x)cos(3) = cos(x)sin(3)
factor out sin(x) on the left

sin(x)(1-cos(3)) = cos(x)sin(3)
square both sides

sin^2(x) (1-cos(3))^2 = cos^2(x)sin^2(3)

since sin^2(x) + cos^2(x) = 1, we can replace
cos^2(x) = 1 - sin^2(x)

We now have

sin^2(x) (1 - cos(3))^2 = (1 - sin^2(x))) sin^2(3)
expand on the right
sin^2(x) (1 - cos(3))^2 = sin^2(3) - sin^2(x)sin^2(3)
solve for sin^2(x)
sin^2(x) (1 - cos(3))^2 - sin^2(x)sin^2(3) = sin^2(3)
factor out sin^2(x)

sin^2(x) [ (1 - cos(3))^2 - sin^2(3) ] = sin^2(3)

sin^2(x) = sin^2(3) / [ (1 - cos(3))^2 - sin^2(3) ]

Looks awful but remember that everything on the right is "just a number"

Square-rooting both sides will give you two results (one positive, the other negative). You will have to add 2*pi to the negative angle x, in order to bring it into the range asked in the question.
2021-04-07 1:13 pm
sin(x + 3) = sin(x)
sin(x)*cos(3) + cos(x)*sin(3) = sin(x)
cos(x)*sin(3) = sin(x) - sin(x)*cos(3)
sin(x)*(1 - cos(3)) = cos(x)*sin(3)
sin(x)/cos(x) = sin(3) / (1 - cos(3))
tan(x) = sin(3) / (1 - cos(3))
x = pi*n + (pi/2) - 1.5, for any integer n
Since we want 0 <= x <= 2*pi, then we have:
x = (pi/2) - 1.5 or x = (3*pi/2) - 1.5

The product of the two solutions is:
((pi/2) - 1.5)((3*pi/2) - 1.5) = 0.75*pi^2 - 3*pi + 2.25
=~ 0.2274253400476392487379381000686
2021-04-07 11:25 am
Since you are looking for decimal approximations, I'll use Newton's Method. We need to get your equation into an expression equal to zero, so:

sin(x + 3) = sin(x)
sin(x + 3) - sin(x) = 0

We can call that our function:

f(x) = sin(x + 3) - sin(x)

We'll need the first derivative.  The second term is simple. The first needs the chain rule:

y = sin(u) and u = x + 3
dy/du = cos(u) and du/dx = 1

dy/dx = dy/du * du/dx
dy/dx = cos(u) * 1
dy/dx = cos(u)
dy/dx = cos(x + 3)

So the first derivative is:

f'(x) = cos(x + 3) - cos(x)

Looking at a graph of the two curves, they meet near 0 and near 3.  So we can use those as the first guesses.

The equation we need is:

x₁ = x₀ - f(x₀) / f'(x₀)

Substitute our expressions to get:

x₁ = x₀ - [sin(x₀ + 3) - sin(x₀)] / [cos(x₀ + 3) - cos(x₀)]

Using x₀ = 0, first, and making sure my calculator is in radians mode:

x₁ = 0 - [sin(0 + 3) - sin(0)] / [cos(0 + 3) - cos(0)]
x₁ = 0 - [sin(3) - 0] / [cos(3) - 1]
x₁ = 0 - (0.14112 - 0) / (-0.989992 - 1)
x₁ = 0 - 0.14112 / (-1.989992)
x₁ = 0 + 0.070915
x₁ = 0.070915

Let's loop this again until we know we have a number good to 4DP:

x₂ = x₁ - [sin(x₁ + 3) - sin(x₁)] / [cos(x₁ + 3) - cos(x₁)]
x₂ = 0.070915 - [sin(0.070915 + 3) - sin(0.070915)] / [cos(0.070915 + 3) - cos(0.070915)]
x₂ = 0.070915 - [sin(3.070915) - sin(0.070915)] / [cos(3.070915) - cos(0.070915)]
x₂ = 0.070915 - (0.0706188 - 0.0708556) / (-0.9975034 - 0.9974866)
x₂ = 0.070915 - (-0.0002368) / (-1.99499)
x₂ = 0.070915 - 0.0001187
x₂ = 0.0707963

Since we only subtracted 0.0001 from the previous guess, this should be good to 4DP.

x₂ = 0.0708

Now let's do the same starting with the guess of 3 for the other answer:

x₁ = x₀ - [sin(x₀ + 3) - sin(x₀)] / [cos(x₀ + 3) - cos(x₀)]
x₁ = 3 - [sin(3 + 3) - sin(3)] / [cos(3 + 3) - cos(3)]
x₁ = 3 - [sin(6) - sin(3)] / [cos(6) - cos(3)]
x₁ = 3 - (-0.279415 - 0.14112) / [0.9601703 - (-0.9899925)]
x₁ = 3 - (-0.420535) / (0.9601703 + 0.9899925)
x₁ = 3 + 0.420535 / 0.969876
x₁ = 3 + 0.433597
x₁ = 3.433597

x₂ = x₁ - [sin(x₁ + 3) - sin(x₁)] / [cos(x₁ + 3) - cos(x₁)]
x₂ = 3.433597 - [sin(3.433597 + 3) - sin(3.433597)] / [cos(3.433597 + 3) - cos(3.433597)]
x₂ = 3.433597 - [sin(6.433597) - sin(3.433597)] / [cos(6.433597) - cos(3.433597)]
x₂ = 3.433597 - [0.1498452 - (-0.287872)] / [0.9887095 - (-0.9576688)]
x₂ = 3.433597 - (0.1498452 + 0.287872) / (0.9887095 + 0.9576688)
x₂ = 3.433597 - 0.4377172 / 1.9463783
x₂ = 3.433597 - 0.224888
x₂ = 3.208709

We'll need at least one more loop:

x₃ = x₂ - [sin(x₂ + 3) - sin(x₂)] / [cos(x₂ + 3) - cos(x₂)]
x₃ = 3.208709 - [sin(3.208709 + 3) - sin(3.208709)] / [cos(3.208709 + 3) - cos(3.208709)]
x₃ = 3.208709 - [sin(6.208709) - sin(3.208709)] / [cos(6.208709) - cos(3.208709)]
x₃ = 3.208709 - [-0.074407 - (-0.067066)] / [0.9972279 - (-0.9977485)]
x₃ = 3.208709 - (-0.074407 + 0.067066) / (0.9972279 + 0.9977485)
x₃ = 3.208709 - (-0.007341) / 1.9949764
x₃ = 3.208709 + 0.007341 / 1.9949764
x₃ = 3.208709 + 0.003680
x₃ = 3.212389

One more should do it:

x₄ = x₃ - [sin(x₃ + 3) - sin(x₃)] / [cos(x₃ + 3) - cos(x₃)]
x₄ = 3.212389 - [sin(3.212389 + 3) - sin(3.212389)] / [cos(3.212389 + 3) - cos(3.212389)]
x₄ = 3.212389 - [sin(6.212389) - sin(3.212389)] / [cos(6.212389) - cos(3.212389)]
x₄ = 3.212389 - [-0.07073718 - (-0.07073722)] / [0.997495 - (-0.997495)]
x₄ = 3.212389 - (-0.07073718 + 0.07073722) / (0.997495 + 0.997495)
x₄ = 3.212389 - 0.00000004 / 1.99499
x₄ = 3.212389 - 0.00000002005
x₄ = 3.212389

Round this to 4DP:

x₄ = 3.2124

Finally, the product of your two roots:

0.0708 * 3.2124 = 0.2274 (rounded to 4DP)
2021-04-10 11:47 pm
 sin(x + 3 )= sin x, 0 ≤ x ≤ 2π to 4 decimal places. 
 sin(3) cos(x) + cos(3) sin(x) = sin(x)
 Solutions:
 x ≈ 2 (3.1416 n + 0.035398), n element Z
 x ≈ 2 (3.1416 n - 0.035398), n element Z
 The product of the two solutions is
 39.4786 n^2 - 0.00501207
2021-04-08 1:55 am
looking at the graph, which sections are parallel from 0 to 2pi?

pi/2 and 3pi/2

go 1.5 (half of 3) on either side of those

sin(pi/2 - 1.5) = sin(pi/2 + 1.5)

x = pi/2 - 1.5
x + 3 = pi/2 + 1.5


sin(x) = sin(x + 3) = 0.07074
x = 3pi/2 - 1.5
x + 3 = 3pi/2 + 1.5

sin(x) = sin(x + 3) = -0.07074

I don't know what you mean by the product of two solutions.
x and x+3 or each x?
2021-04-07 10:36 am
how can sin (x+3) ever equal sin x?? I've not taken a math class in forever, so I'm really curious if this is actually what you were asked?
2021-04-07 10:33 am
Well, you have it all wrong. Pi. Is equal to 78 divided by 5, and 0 is not less than the equation.


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