e^(x+y)=e^x*e^y(同底数幂相乘，底数不变，指数相加)e^(-y/x)=1/e^(y/x)e^(x-y)=e^x/e^y(同底数幂相除，底数不变，指数相减)
e^(x+y)=e^x*e^ye^(-y/x)=1/e^(y/x)e^(x-y)=e^x/e^y
