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  1. GATE CS
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  3. Engineering Mathematics

The value of int_0^pi x * cos^2(x) dx correct to three decimal places (assuming that pi = 3.14) is ______.

GATE 2018 · Engineering Mathematics · Integration · medium

Answer: 2.4649 (approximately 2.465 to three decimal places with pi = 3.14)

  1. Apply power-reduction identity: int_0^pi x*cos^2(x)dx = (1/2)*int_0^pi x dx + (1/2)*int_0^pi x*cos(2x) dx
  2. Evaluate first part: (1/2) * pi^2/2 = pi^2/4
  3. Evaluate second part by IBP: (1/2)*[x*sin(2x)/2 + cos(2x)/4]_0^pi = (1/2)*[pi*sin(2pi)/2 + cos(2pi)/4 - 0 - cos(0)/4] = (1/2)*[0 + 1/4 - 1/4] = 0
  4. Combine and substitute pi = 3.14: pi^2/4 = (3.14)^2/4 = 9.8596/4 = 2.4649