定积分:分母是X乘上根号下(x^2+4),分子是一.上限是2根号3,下限是2

换元法:
x=2tanθ,由于x∈[2,2√3],从而θ∈[π/4,π/3],
√(x²+4)=2/cosθ,dx=2dθ/cos²θ,
从而∫[2,2√3]dx/(x√(x²+4))
=1/2∫[π/4,π/3]dθ/sinθ
=-1/4*ln2+1/2*ln(2+√2)-1/4*ln3
凑微分法:
∫[2,2√3]dx/(x√(x²+4))
=∫[2,2√3]dx/(x²√(1+4/x²))
=∫[2,2√3]x^(-2)dx/√(1+4/x²)
=-1/2∫[2,2√3]d(2/x)/√(1+(2/x)²)
=-1/2ln(2/x+√(1+(2/x)²))[2,2√3]
=-1/4*ln2+1/2*ln(2+√2)-1/4*ln3

换元 x= 2 tga
利用 (tg a)^2 + 1=(sec a)^2
tg a= sin a/cos a
两个式子带进去凑微分。

根据公式∫1/(a^2+x^2)dx=(1/a)*arctan(x/a)+c
∫1/(2^2+x^2)dx=(1/2)*arctan(x/2)+c
其定积分=(1/2)*arctan(x/2)|2√3-(1/2)*arctan(x/2)|2
=(1/2)*arctan√3-(1/2)*arctan1
=(1/2)(mπ+π/6)-(1/2)(kπ+π/4)
=(1/2)(nπ-π/12)

∵设x=2tant,则当x=2√3时，t=π/3.当x=2时，t=π/4
∴原式=1/2∫(π/4,π/3)dt/sint (∫(π/4,π/3)表示从π/4到π/3积分)
=1/2∫(π/4,π/3)sintdt/sin²t
=1/2∫(π/4,π/3)d(cost)/(cos²t-1)
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