Bài $14$.
$a$) `8/2 - |7/9 - x| = -1/5`
`⇔ |7/9 - x | = 21/5`
`⇒` \(\left[ \begin{array}{l}x=\dfrac{-154}{45}\\x=\dfrac{224}{45}\end{array} \right.\)
Vậy $x$ $∈$ `{{-154}/{45};{224}/{45}}`
$b$) `(x-1/4)^2-{25}/{64}=0`
`⇔ (x-1/4)^2 = {25}/{64}`
`⇔ x-1/4 = ± 5/8`
`⇒` \(\left[ \begin{array}{l}x=\dfrac{7}{8}\\x=\dfrac{-3}{8}\end{array} \right.\)
Vậy $x$ $∈$ `{7/8;-3/8}`
$c$) `(x-1/4)^2+17/64=21/32`
`⇔ (x-1/4)^2 = {25}/64`
`⇔ x-1/4 = ± 5/8`
`⇒` \(\left[ \begin{array}{l}x=\dfrac{7}{8}\\x=\dfrac{-3}{8}\end{array} \right.\)
Vậy $x$ $∈$ `{7/8;-3/8}`
$d$) `2/5. x+1/3=1/5`
`⇔ 2/5 . x = -2/{15}`
`⇔ x = -1/3`
Vậy `x=-1/3`
$e$) ` 4/9-5/3 .x=-2`
`⇔ 5/3 . x = {22}/9`
`⇔ x = {22}/{15}`
Vậy `x = {22}/{15}`
$f$) `5/7 :x-3= -2/7`
`⇔ 5/7 :x = {19}/7`
`⇔ x = 5/{19}`
Vậy `x = 5/{19}`
Bài $15$.
`A = 3/{1.3} + 3/{3.5} + 3/{5.7} + .... + 3/{199.201}`
`⇔ A/3 = 1/{1.3} + 1/{3.5} + 1/{5.7} + .... +1/{199.201}`
`⇔ A . 2/3 = 2/{1.3} + 2/{3.5} + 2/{5.7} + .... +2/{199.201}`
`⇔ A . 2/3 = 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ..... + 1/{199} - 1/{201}`
`⇔ A . 2/3 = {200}/{201}`
`⇔ A = {100}/{67}`
`B = 5/{1.3} + 5/{3.5} + 5/{5.7} + .... + 5/{201.203}`
`⇔ B/5 = 1/{1.3} + 1/{3.5} + 1/{5.7} + .... + 1/{201.203}`
`⇔ B . 2/5 = 2/{1.3} + 2/{3.5} + 2/{5.7} + .... +2/{201.203}`
`⇔ B . 2/5 = 1/1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + .... + 1/{201} - 1/{203}`
`⇔ B . 2/5 = {202}/{203}`
`⇔ B = {505}/{203}`
