\[\begin{array}{l} Bai\,\,40 - sgk\,\,trang\,\,\,19\\ a)\,\,15.91,5 + 150.0,85\\ = 15.91,5 + 15.8,5\\ = 15\left( {91,5 + 9,5} \right) = 15.100 = 1500.\\ b)\,\,x\left( {x - 1} \right) - y\left( {1 - x} \right)\\ = x\left( {x - 1} \right) + \left( {x - 1} \right)y\\ = \left( {x - 1} \right)\left( {x + y} \right).\\ Thay\,\,\,\,x = 2001;\,\,\,y = 1999\,\,\,vao\,\,\,bieu\,\,thuc\,\,ta\,\,duoc:\\ \left( {2001 - 1} \right)\left( {2001 + 1999} \right) = 2000.4000 = 8000000. \end{array}\]

a) 15.91,5 + 150.0,85

= 15.91,5 + 15.10.0,85

= 15.91,5 + 15.8,5

= 15(91,5 + 8,5)

= 15.100

= 1500

b) x(x – 1) – y(1 – x)

= x(x – 1) – y[–(x – 1)]

= x(x – 1) + y(x – 1)

= (x – 1)(x + y)

Tại x = 2001, y = 1999, giá trị biểu thức bằng:

(2001 – 1)(2001 + 1999) = 2000.4000 = 8000000