Giúp mình với ạ. Vote 5* + CTLHN

$a$) `A = 81.[{12-{12}/{7} - {12}/{289} - {12}/{85}}/{4-{4}/{7} - {4}/{289} - {4}/{85}} : {5+{5}/{13} + {5}/{169} + {5}/{91}}/{6+{6}/{13} + {6}/{169} + {6}/{91}}] . {158158158}/{711711711}`

`⇔ A = 81.[{12.(1-{1}/{7} - {1}/{289} - {1}/{85})}/{4(1-{1}/{7} - {1}/{289} - {1}/{85})} : {5(1+{1}/{13} + {1}/{169} + {1}/{91})}/{6(1+{1}/{13} + {1}/{169} + {1}/{91})}] . {158}/{711}`

`⇔ A = 81.[{12}/{4} : {5}/{6}] . {2}/{9}`

`⇔ A = 81.(3 . 6/5). 2/9`

`⇔ A = 81. {18}/5 . 2/9`

`⇔ A = 324/5`.

$b.1$) `-2/3(x-1/4) = 1/3.(2x-1)`

`⇔ -2/3x + 1/6 = 2/3x - 1/3`

`⇔ 1/6 - 2/3x = 2/3x - 1/3`

`⇔ 1/6 + 1/3 = 2/3x + 2/3x`

`⇔ 1/2 = 4/3 x`

`⇔ x = 3/8`.

$b.2$) `1/5 . 2^x + 1/3 . 2^{x+1} = 1/5 . 2^7 + 1/3 . 2^8`

`⇔ 1/5 . 2^x + 1/3 . 2^{x+1} = 1/5 . 2^7 + 1/3 . 2^{7+1}`

`⇔ x = 7`