c= 1995 x 1995
d = ( 1991 + 4 ) x ( 1999 - 4 )
nên : c = 1995 x 1995 = d =1991 x 1999
C = 3980025
D = 3980009
Vì 3980025 > 3980009
suy ra C > D
C=1995 x 1995 =(1991 + 4) x 1995
=1991 x 1995 + 4 x 1995
D=1991 x 1999 =1991 x (1995 + 4)
=1991 x 1995 + 1991 x 4