Đáp án:
a) 3 . x . (2 - 7)
= 3 . x . (-5)
= 3 . (-5) . x
= -15 . x
b) 2015² - 2015 . 4018 + 2014²
= 2015 . 2015 - 2015 . 4018 + 2014 . 2014
= 2015 . 2015 - 2015 . 4018 + (2015 - 1) . 2014
= 2015 . 2015 - 2015 . 4018 + 2015 . 2014 - 1 . 2014
= 2015 . (2015 - 4018 + 2014) - 2014
= 2015 . 11 - 2014
= 2015 . 11 - (2015 - 1)
= 2015 . 11 - 2015 + 1
= 2015 . (11 - 1) + 1
= 2015 . 10 + 1
= 20151
Chúc học tốt!!!