a+1/a=(根号a-根号a分之1)的平方-1,即(根号a-根号a分之1)的值为正负根号下5
(SQR(a)-1/SQR(a))^2=a+1/a-2=4-2＝2又因为0所以SQR(a)-1/SQR(a)＝负根号2
a+1/a=4得， a^2+1=4a令SQR(a)-1/SQR(a)=A则A^2=(a^2+2a+1)/a=6a/a=6SQR(a)-1/SQR(a)=正负根号6
