a) ( 45 + 12 ) + ( 2012 - 120 - 45 )
= 45 + 12 + 2012 - 120 - 45
= 57 + 2012 - 120 - 45
= 2069 - 120 - 45
= 1976 - 45
= 1931
b) ( 1890 - 19 - 5 ) - ( 19 - 5 + 1890 )
= 1890 - 19 - 15 - 19 - 5 + 1890
= 1871 - 15 - 19 - 5 + 1890
= 1856 - 19 - 5 + 1890
= 1837 - 5 + 1890
= 1832 + 1890
= 3722
a, (45+12) + (2012-120-45)
= 45+12+2012-120-45
=(45-45) + 2012-120+12
=0+1892+12
=1904
b, (1890-19-5) - (19-5+1890)
=1890-19-5+19+5-1890
=(1890-1890)+(-19+19)+(-5+5)
=0
a,(45+12)+(2012-120-45)
=45+12+2012-120-45
=57+2012-120-45
=2069-120-45
=1949-45
=1904
Cứ thế làm tiếp nhé