f(x) = tanx - x, f(nπ) = 0 - nπ = -nπ < 0,记 x→- 表示 x 从左边趋近于 nπ+π/2，则lim- (tanx-x)= lim- tanx - lim- x= +∞ - (nπ+π/2) = +∞ > 0
