ทำตรงๆแบบไม่ต้องคิดมาก
$$\frac{(20)(\frac{21}{2})(\frac{22}{3})...(\frac{38}{19})(\frac{39}{20})...(\frac{110}{91})}{(92)(\frac{93}{2})(\frac{94}{3}).. .(\frac{110}{19})}$$ $$=\frac{20\cdot21\cdot22\cdot...\cdot110}{2\cdot3\cdot...\cdot91}\times\frac{2\cdot3...\cdot19}{92\cdot93\cdot...\cdot110}$$ $$=\frac{20\cdot21\cdot...\cdot91}{20\cdot21\cdot...\cdot91}=1$$
|