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) \(x(x - 1) - y(1 - x) \)
\(= x(x - 1) - y[-(x - 1)]\)
\(= x(x - 1) + y(x - 1)\)
\(= (x - 1)(x + y)\)
Thay \(x = 2001, y = 1999\) ta được:
\((2001 - 1)(2001 + 1999) \)
\(= 2000 . 4000 = 8000000\)
