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The Fourier series expansion of x3 in the interval −1 ≤ x < 1 with periodic continuation has
  • a)
    only sine terms
  • b)
    only cosine terms
  • c)
    both sine and cosine terms
  • d)
    only sine terms and a non-zero constant
Correct answer is option 'A'. Can you explain this answer?
Most Upvoted Answer
The Fourier series expansion of x3 in the interval −1 ≤ x <...
The Fourier series expansion of x^3 in the interval [-π, π] is given by:

x^3 = a0/2 + Σ (an*cos(nx) + bn*sin(nx))

where the coefficients are calculated as follows:

a0 = (1/π) ∫[-π, π] x^3 dx = 0 (since x^3 is an odd function)

an = (1/π) ∫[-π, π] x^3*cos(nx) dx = 0 (since the integrand is an odd function)

bn = (1/π) ∫[-π, π] x^3*sin(nx) dx

To calculate bn, we can use integration by parts:

bn = (1/π) ∫[-π, π] x^3*sin(nx) dx
= (1/π) * [-x^3*cos(nx)/(n) + ∫[π, -π] 3x^2*cos(nx)/(n) dx]
= (1/π) * [-x^3*cos(nx)/(n) + 3/n * ∫[π, -π] x^2*cos(nx) dx]

Now, we can use integration by parts again to evaluate the integral:

bn = (1/π) * [-x^3*cos(nx)/(n) + 3/n * (-x^2*sin(nx)/(n) + ∫[π, -π] 2x*sin(nx) dx)]
= (1/π) * [-x^3*cos(nx)/(n) - 3/n^2 * x^2*sin(nx) + 6/n^2 * ∫[π, -π] x*cos(nx) dx]
= (1/π) * [-x^3*cos(nx)/(n) - 3/n^2 * x^2*sin(nx) + 6/n^2 * (-x*sin(nx)/(n) + ∫[π, -π] sin(nx) dx)]
= (1/π) * [-x^3*cos(nx)/(n) - 3/n^2 * x^2*sin(nx) + 6/n^3 * x*sin(nx) - 6/n^3 * cos(nx)]

Therefore, the Fourier series expansion of x^3 in the interval [-π, π] is:

x^3 = Σ (bn*sin(nx))

where bn = (1/π) * [-x^3*cos(nx)/(n) - 3/n^2 * x^2*sin(nx) + 6/n^3 * x*sin(nx) - 6/n^3 * cos(nx)]
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Community Answer
The Fourier series expansion of x3 in the interval −1 ≤ x <...
f(x) = x3
find f(x) is even or odd
put x = -x
f(-x) = - x3
f(x) = -f(-x) hence it is odd function
for odd function, ao = an = 0 
Fourier Series for odd function has only bn term

Hence only sine terms are left in Fourier expansion of x3
Additional Information
Fourier Series
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The Fourier series expansion of x3 in the interval −1 ≤ x < 1 with periodic continuation hasa)only sine termsb)only cosine termsc)both sine and cosine termsd)only sine terms and a non-zero constantCorrect answer is option 'A'. Can you explain this answer?
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