Câu 1)
`\frac{1}{1 xx2}` + `\frac{1}{2xx3}`+ .....+`\frac{1}{99xx100}`
Ta có :
`\frac{1}{1 xx2}` = `1/1 - 1/2`
`\frac{1}{2xx3}` = `1/2 - 1/3`
.............................
`\frac{1}{99xx100}`= `1/99 - 1/100`
⇒ `\frac{1}{1 xx2}` + `\frac{1}{2xx3}`+ .....+`\frac{1}{99xx100}`
= `1/1 - 1/2` + `1/2 - 1/3 + .... + `1/99 - 1/100`
= ` 1/1 - 1/100`
`= 99/100`
Câu 2)
`|x+3/4|= 1/2`
⇒ `x + 3/4 = 1/2` hoặc `x + 3/4 = -1/2`
`x = 1/2 - 3/4` hoặc `x = -1/2 - 3/4`
`x = 2/4 - 3/4` hoặc `x = -2/4 - 3/4`
`x = -1/4` hoặc `x = -5/4`
