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又tan x=3tan y
则tan（x-y）=(tanx-tany)/(1+tanx*tany)
=2tany/(1+3tany*tany)
=2/(1/tany+3tany)
又1/tany+3tany≥2/根号3
则tan（x-y）≤根号3/3
则x-y≤π/6
即x-y≤30º