1 a) Rút gọn A:
A = x/(x^2 - 25) - (x-5)/(x^2 + 5x)
A = x/((x+5)(x-5)) - (x-5)/x(x+5)
A = (x(x-5) - (x-5)(x+5))/(x(x+5)(x-5))
A = (x^2 - 5x - x^2 - 5x + 25)/(x(x+5)(x-5))
A = (-10x + 25)/(x(x+5)(x-5))
A = -10/(x+5)

Rút gọn B:
B = (2x-5)/(x^2 + 5x) + (x+3)/(5-x)
B = (2x-5)/(x(x+5)) + (x+3)/(5-x)
B = (2x-5)/(x(x+5)) - (x+3)/(x-5)
B = [(2x-5)(x-5) - (x+3)(x+5)]/(x(x+5)(x-5))
B = (2x^2 - 10x - 5x + 25 - x^2 - 5x + 3x + 15)/(x(x+5)(x-5))
B = (x^2 - 17x + 40)/(x(x+5)(x-5))
B = (x-8)(x-5)/(x(x+5)(x-5))
B = (x-8)/(x+5)

2 b) Tính P = A:B
P = A/B
P = (-10/(x+5)) / ((x-8)/(x+5))
P = -10(x+5)/(x+5)(x-8)
P = -10/(x-8)

3 c) Tính P tại x=0, x=4
- Khi x = 0:
P = -10/(0-8) = -10/-8 = 5/4
- Khi x = 4:
P = -10/(4-8) = -10/-4 = 5/2

Vậy kết quả là:
- P = 5/4 khi x = 0
- P = 5/2 khi x = 4