cosx的值域是[-1,1]π≈3.14 π/2≈1.57所以-tan1tan11是弧度制
-1<=cosx<=1tanx在(-π/2，π/2)是增函数而[-1,1]在(-π/2，π/2)内所以值域[-tan1,tan1]
[tan(-1),tan1]
