x²+y²+z²=2
x=y
∴2x²+z²=2
所以L的参数方程为：x=y=cosθ,z=√2sinθ,0≤θ≤2π
ds=√(x'²+y'²+z'²)dθ=√2dθ
∫|y|ds=∫(0→2π)|cosθ|√2dθ
=∫(0→π/2)cosθ√2dθ-∫(π/2→3π/2)cosθ√2dθ+∫(3π/2→2π)cosθ√2dθ
=√2-√2×(-1-1)+√2×[0-(-1)]
=4√2