反向变量替换，配方 sec=1cos   tan=sincos\large sec = \frac{1}{cos} \ \ \ tan = \frac{sin}{cos}sec=cos1​   tan=cossin​ csc=1sin   cot=cossincsc = \frac{1}{sin} \ \ \ cot = \frac{cos}{sin}csc=sin1​   cot=sincos​ sec2=1+tan2sec^2 = 1+tan^2sec2=1+tan2 tan′=sec2ta