\(1990\times1990-1992\times1988\)

 \(=\left(1988+2\right)\times1990-\left(1990+2\right)\times1988\)

  \(=1988\times1990+2\times1990-1990\times1988-2\times1988\)

  \(=2\times1990-2\times1988\)

  \(=2\times\left(1990-1988\right)\)

  \(=2\times2\)

   \(=4\)

pạn -1 vào mỗi phân số là xong. Rùi ra x\(\frac{x-2015}{1986}\)+\(\frac{x-2015}{1988}\)+ \(\frac{x-2015}{1990}\)+...+\(\frac{x-2015}{x1996}\)-\(\frac{x-2015}{29}\)-\(\frac{x-2015}{27}\)-...\(\frac{x-2015}{19}\)=0

<=>(x-2015)(\(\frac{1}{1986}\)+\(\frac{1}{1988}\)+... -\(\frac{1}{19}\))=0...(mà \(\frac{1}{1986}\)+...- \(\frac{1}{19}\) khác 0)

=>x-2015=0

<=> x=2015

1990x1990-1992x1988

=4:))

=4

C = 1988 . 1990 + 2 - 1990 - 1990 - 1988 - 2 . 1988

C = ( 2 . 1990 - 2 . 1988 ) + ( 1990 . 1988 - 1988 . 1990 )

C = 2 . ( 1990 - 1988 ) + 0

C = 2 + 2 + 0

C = 4

19991999 . 1998 - 19981998.1999

= 1999 . 1001 . 1998 - 1998 . 1001 . 1999

= 0

tick nhé bạn

19991999 . 1998 - 19981998.1999

= 1999 . 1001 . 1998 - 1998 . 1001 . 1999

= 0

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

1990 * ( 1992 - 1990 ) 

1990 * 2

3980

Ta có: 1990.1990 = (1988+2). 1990

                          = 1988.1990+2.1990

1992.1988= (1990+2).1988=1990.1988+2.1988

=> 1990.1990-1992.1988=1988.1990+2.1990-1990.1988-2.1988=2(1990-1988)=2.2=4

=-19916

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