a)

`{(x/2 = y/3),(x + y - 10 = 0):}`

`<=> {(3x - 2y = 0),(x + y = 10):}`

`<=> {(3x - 2y = 0),(2x + 2y = 20):}`

`<=> {(5x = 20),(x + y = 10):}`

`<=> {(x = 4),(4 + y = 10):}`

`<=> {(x = 4),(y = 6):}`

Vậy `(x;y) = (4;6)`

b)

`A = sqrt((1-sqrt(2))^2 ) + sqrt(72)`

`A = |1 - sqrt(2)| + sqrt(6^2 .2)`

`A = sqrt(2) - 1 + 6sqrt(2)`

`A = 7sqrt(2) - 1`

c)

ĐKXĐ: `x >= 0; x \ne 1`

`B = (     (sqrt(x)-2)/(x-1)- (sqrt(x) + 2)/(x + 2sqrt(x) + 1)     ) . (x^2 - 2x + 1)/2`

`B= [        (sqrt(x) - 2)/((sqrt(x) + 1)(sqrt(x) - 1)) - (sqrt(x) + 2)/((sqrt(x) + 1)^2) ]. ((x - 1)^2)/2`

`B = [((sqrt(x) - 2)(sqrt(x) + 1))/((sqrt(x) + 1)^2(sqrt(x)-1)) - ((sqrt(x)+2)(sqrt(x) - 1))/((sqrt(x) + 1)^2(sqrt(x)-1))] . ((x-1)^2)/2`

`B = (   x + sqrt(x) - 2sqrt(x) - 2 - x + sqrt(x) - 2sqrt(x) + 2      )/((sqrt(x) + 1)^2(sqrt(x)-1)) . ((x-1)^2)/2`

`B = (-2sqrt(x))/((sqrt(x) + 1)^2(sqrt(x)-1))  .  ((x-1)^2)/2`

`B = (-sqrt(x).(x-1)^2)/((sqrt(x) + 1)^2(sqrt(x)-1))`

`B = (-sqrt(x) [(sqrt(x) + 1)(sqrt(x) - 1)]^2 )/((sqrt(x) + 1)^2(sqrt(x)-1))`

`B = (-sqrt(x) [(sqrt(x) + 1)]^2 [(sqrt(x) - 1)]^2)/((sqrt(x) + 1)^2(sqrt(x)-1))`

`B = -sqrt(x) . (sqrt(x) - 1)`

`B = sqrt(x) - x`