4. Find the arc length for the curve y = x^2 - lnx taking from x = 1
to x = 3
y'(x) = 2x - 1/x
Length = ∫ds = ∫√[1+(y')^2] dx from 1 to 3
似乎沒辦法積出來？也不太容易改成以 dy 積分？
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