A=1991x1999=(1995-4)1999=1995x1999-4x1999
B=1995x1995=1995x(1999-4)=1995x1999-1995x4>1995x1999-4x1999=A
vậy A<B
A = 1991 x 1999 B = 1995 x 1995
A = 1991 x ( 1995 + 4 ) B = ( 1991 + 4 ) x 1995
A = 1991 x 1995 + 1991 x 4 B = 1991 x 1995 + 1995 x 4
Vì 1991 x 4 < 1995 x 4 => 1991 x 1999 < 1995 x 1995
Vậy A < B
A=1991x1999=(1995-4)1999=1995x1999-4x1999
B=1995x1995=1995x(1999-4)=1995x1999-1995x4>1995x1999-4x1999=A
vậy A<B