tìm y biết y+1/2+1/4+1/8+1/16+1/32+1/64=1

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

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

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

` 2A - A = (1+1/2+1/4+1/8+1/16+1/32) - (1/2+1/4+1/8+1/16+1/32+1/64)` 

` A = 1 - 1/64` 

` A = 63/64` 

`=> y + 63/64 = 1` 

` y = 1 - 63/64` 

` y = 1/64` 

Vậy `y=1/64`