a) 15.91,5+150.0,8515.91,5+150.0,85
=15.91,5+15.8,5=15.91,5+15.8,5
=15(91,5+8,5)=15(91,5+8,5)
=15.100=15.100
=1500=1500
b) x(x−1)−y(1−x)x(x−1)−y(1−x)
=x(x−1)−y[−(x−1)]=x(x−1)−y[−(x−1)]
=x(x−1)+y(x−1)=x(x−1)+y(x−1)
=(x−1)(x+y)=(x−1)(x+y)
Thay x=2001,y=1999x=2001,y=1999 vào biểu thức, ta được:
(2001–1)(2001+1999)=2000.4000=8000000