y=2-sinx/cosx+1ycosx+y=2-sinxsinx+ycosx=2-y√(1+y²)sin(x+∅)=2-ysin(x-∅)=(2-y)/√(1+y²)所以 |(2-y)/√(1+y²)|≤1|2-y|≤√(1+y²)两边平方y²-4y+4≤1+y²4y≥3y≥3/4...