Đáp án:
Giải thích các bước giải:
`b) 4/3 .x+3/4=7/8 `
`4/3 .x+3/4 .x=7/8 `
`x. (4/3+3/4)=7/8 `
`x. 23/12=7/8 `
` x = 7/8 : 23/12`
` x = 21/46 `
`c) x+1/-2=-32/x+1`
` (x+1).(x+1)=-32.(-2)`
`x.(1+1) = 64`
`x.2 = 64`
`x = 64 :2`
`x = 32 `
5.
`A = 1/2 - 1/3 + 1/4 - 1/5 +... + 1/2018 - 1/2019 >1/5 `
`A = 1/6 + 1/20 +...+ 1/2018 - 1/2019 > 1/5 `
`A = 1/(1.2) + 1/(2.10) +.....+ 1/2018 - 1/2019 > 1/5 `
`A = 1:2( 1/1 - 1/2 + 1/2 - 1/10 +....+ 1/2018 - 1/2019) > 1/5`
`A=1/2( 1/1 - 1/2019)`
`A=1/2 . 2018/2019`
`A=1009/2019`
Do ` 1009/2019 = 0,49975...`
Mà ` 1/5 = 0,2 `
Do ` 0,49 > 0,2`
`=> A > 1/5(đpcm)`
