a) A=x/√x + 1 (đk x≥0)

Thay x=1/4

A=1/4 : √1/4 +1

=   1/4 : 1/2 + 1

= 1/4 : 3/2

= 1/4 . 2/3

=2/12 = 1/6

b) B=(3/√x + 1 ) + (1/1 - √x) + (x+5 / x-1) (đk x≥0;x≠1)

= (3/√x + 1 ) - ( 1/√x -1 ) + [x+5/(√x+1).(√x-1) ]

= [3.(√x-1) /(√x+1). (√x-1) ] - [(√x+1) / (√x+1).(√x-1) ] + [x+5/(√x+1).(√x-1) ]

= [3(√x-1) - (√x+1) + (x+5) ]/(√x+1). (√x-1)

= (3√x - 3 -√x -1 + x + 5 ) /(√x+1).(√x-1)

= (x +2√x +1) /(√x+1).(√x-1)

=(√x+1)^2/(√x+1).(√x-1)

=(√x+1)/(√x-1)

c) P= A.B

= (x/√x+1).[(√x+1)/(√x-1)]

=x/√x-1

P≤4 suy ra x/√x-1 ≤ 4

1/3

10;33, 34%