F（X）+2F（X分之1）=X
用1/X替换X得
F（1/X）+2F（X）=1/X
联立解方程组：
F（X）+2[1/X-2F（X）]=X
F（X）+2/X-4F（X）=X
-3F（X）=X-2/X
F（X）=2/(3X)-X/3