X2+9的平方之一的不定积分

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X2+9的平方之一的不定积分

x = 3tanθ、dx = 3sec^2(θ) dθ
∫ dx/(x^2 + 9)^2
= ∫ [3sec^2(θ)]/[9sec^2(θ)]^2 dθ
= ∫ [3sec^2(θ)]/[81sec^4(θ)] dθ
= (1/27)∫ cos^2(θ) dθ
= (1/54)∫ (1 + cos2θ) dθ
= (1/54)θ + (1/108)sin2θ + C
= (1/54)θ + (1/54)sinθcosθ + C
= (1/54)arctan(x/3) + (1/54) * x/√(x^2 + 9) * 3/√(x^2 + 9) + C
= (1/54)arctan(x/3) + x/[18√(x^2 + 9)] + C

X2+9的平方之一的不定积分表达式没明白