1，C；y^2=2x
l；x-y-3=0
2,联立直线曲线
t^2-4t-3=0
M(x1,y1),N(x2,y2)
x1+x2=4,x1x2=-3
lPMl^2+lPNl^2=(x1+x2)^2-2x1x2=22
lPMl+lPNl=√(x1+x2)^2-4x1x2=2√7
lPMl*lPNl=-x1x2=3
lMNl=lPMl+lPNl=2√7
1/lPMl+1/lPNl=lPMl+lPNl/lPMl*lPNl=2√7/3
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