Question

tính nhanh: 1990x1990-1992x1988

Answer 1

1990x1990 – 1992x1988  =

1990x1991 – 1990 – 1991x1988 – 1988 =

1991 x (1990 – 1988) – 1990 - 1988 =

1991 x 2 –  1988 – 1988 – 2 =

1991 x 2 – 1988 x 2 – 2 =

2 x (1991 – 1988) – 2 =

2 x 3 – 2 = 4

Answer 2

1990x1990-1992x1988 = 4

Answer 3

1990.1990-1992.1988

=1990.1991.1990-1991.1988-1988

1991.[1990.1988]-1990.1988

=1991.2-1988.2-2

2.[1991-1988]-2

=2.3-2=4

Answer 4

1990x1990-1992×1988

=1990×(1988 + 2)-(1990 + 2)×1988

=(1990×1988+1990×2)-(1990×1988+2×1988)

=1990×1988+1990×2-1990×1988-2×1988

=1990×2-2×1988

=2×(1990 - 1988)

=2×2 

=4