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∵mx+3-nx−3=8xx2−9,∴m(x−3)−n(x+3)x2−9=8xx2−9,即:(m−n)x−3(m+n)x2−9=8xx2−9∴m−n=8m+n=0,即m=4,n=-4.∴mn=-16.故選D.
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