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Trigonometri Question (HELP)

  • Thread starter Bob19
  • Start date
71
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Hi

I have this here Trigonometri function:

F(x) = Ax cos(x) + Bx sin(x) + C cos(x) + D sin(x)


where F'(x) = x cos(x)

I'm suppose to find A, B, C , D any hints or idears on how I do that?

Sincerely
Bob
 

Answers and Replies

LeonhardEuler
Gold Member
858
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Take the derivative of F(x). You know that F'(x)=x cos(x). This means the coefficient of (x cos(x)) should be 1. What can you say about the coefficients of all other terms?
 
71
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LeonhardEuler said:
Take the derivative of F(x). You know that F'(x)=x cos(x). This means the coefficient of (x cos(x)) should be 1. What can you say about the coefficients of all other terms?
Since its trigonometry then A,B,C,D can't higher than one?

F'(x) = A (cos(x) - x *sin(x)) + B(x *cos(x) + sin(x)) + C ( -sin(x)) + D(cos(x) = x cos(x)

But how I proced from here ?

/Bob
 
TD
Homework Helper
1,020
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Bob19 said:
Since its trigonometry then A,B,C,D can't higher than one?
No, these are just coefficients so they can be higher than 1.
It's cos(a) (and sin(a))) which are bounded between -1 and 1.

Bob19 said:
F'(x) = A (cos(x) - x *sin(x)) + B(x *cos(x) + sin(x)) + C ( -sin(x)) + D(cos(x) = x cos(x)
Your derivative seems correct, now just check what the coefficients have to be to get xcos(x).
 
71
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The best way of doing this is that to make the left side of the expression shorter ?

/Bob

TD said:
No, these are just coefficients so they can be higher than 1.
It's cos(a) (and sin(a))) which are bounded between -1 and 1.



Your derivative seems correct, now just check what the coefficients have to be to get xcos(x).
 
TD
Homework Helper
1,020
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Well, we have:

[tex]\begin{array}{l}
F\left( x \right) = Ax\cos x + Bx\sin x + C\cos x + D\sin x \\
F'\left( x \right) = A\left( {\cos x - x\sin x} \right) + B\left( {\sin x + x\cos x} \right) - C\sin x + D\cos x \\
\end{array}[/tex]

Rearranging gives:

[tex]F'\left( x \right) = \left( {A + D} \right)\cos x + \left( {B - C} \right)\sin x - Ax\sin x + Bx\cos x[/tex]

We want this to equal [itex]F'\left( x \right) = x\cos x[/itex]

So:

[tex]\left( {A + D} \right)\cos x + \left( {B - C} \right)\sin x - Ax\sin x + Bx\cos x = x\cos x[/tex]

That should give a very simple system for the unknown coefficients A -> D
 
71
0
TD said:
Well, we have:

[tex]\begin{array}{l}
F\left( x \right) = Ax\cos x + Bx\sin x + C\cos x + D\sin x \\
F'\left( x \right) = A\left( {\cos x - x\sin x} \right) + B\left( {\sin x + x\cos x} \right) - C\sin x + D\cos x \\
\end{array}[/tex]

Rearranging gives:

[tex]F'\left( x \right) = \left( {A + D} \right)\cos x + \left( {B - C} \right)\sin x - Ax\sin x + Bx\cos x[/tex]

We want this to equal [itex]F'\left( x \right) = x\cos x[/itex]

So:

[tex]\left( {A + D} \right)\cos x + \left( {B - C} \right)\sin x - Ax\sin x + Bx\cos x = x\cos x[/tex]

That should give a very simple system for the unknown coefficients A -> D
Thanks then A = 0 , D = 0, B = C = 1

Right ?

/Bob
 

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