tính nhanh : ( 1-1/2 ) x ( 1-1/3) x ( 1-1/4 ) x ....x ( 1-1/99) x ( 1- 1/100)
$(1-\frac{1}{2})(1-\frac{1}{3})...(1-\frac{1}{99})(1-\frac{1}{100})$
$=\frac{1}{2}.\frac{2}{3}...\frac{98}{99}.\frac{99}{100}$
$=\frac{1.2.3...99}{2.3...99.100}=\frac{1}{100}$