cứu với !!! a, $\frac{254 x 399 x145}{254 x 3999 x 253}$ b, $\frac{1}{2}$ + $\frac{1}{4}$ + $\frac{1}{8}$ + $\frac{1}{16}$ $\frac{1}{32}$ + $\frac{1}{64}$

`a)` `frac{254 × 399 × 145}{254 × 3999 × 253}`

`= frac{1 × 399 × 145}{1 × 3999 × 253}`

`= frac{57855}{1011747}`

`= frac{57855}{1011747}`

`b)` `1/2 + 1/4+1/8 + 1/16 + 1/32 + 1/64`

`= 32/64 + 16/64 + 8/64 + 4/64 + 2/64 + 1/64`

`= \frac{32 + 16 + 8 + 4 + 2 + 1}{64}`

`= \frac{63}{64}`