Câu 7. (3,0 điểm) Tìm a, biết: (1 - $\frac{1}{4}$) × (1 - $\frac{1}{5}$) × (1 - $\frac{1}{6}$) × (1 - $\frac{1}{7}$) × (1 - $\frac{1}{8}$) × (1 - $\frac{1}{9}$) = $\frac{a}{75}$

Đáp án + Giải thích các bước giải:

`(1-1/4)xx(1-1/5)xx(1-1/6)xx(1-1/7)xx(1-1/8)xx(1-1/9)=a/75`

`=>(4/4-1/4)xx(5/5-1/5)xx(6/6-1/6)xx(7/7-1/7)xx(8/8-1/8)xx(9/9-1/9)=a/75`

`=>3/4xx4/5xx5/6xx6/7xx7/8xx8/9=a/75`

`=>(3xx4xx5xx6xx7xx8)/(4xx5xx6xx7xx8xx9)=a/75`

`=>1/3=a/75`

`=>3a=75`

`=>a=75:3`

`=>a=25`

Vậy `a=25`