1990 x 1990 - 1992 x 1988 = ?
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1990 x 1990 - 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
Ủng hộ mk nha ^_-
1990.1990-1992.1998
=(1992-2).(1988+2)-(1992.1988)
=2.2
=4
![](https://rs.olm.vn/images/avt/0.png?1311)
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
![](https://rs.olm.vn/images/avt/0.png?1311)
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
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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
![](https://rs.olm.vn/images/avt/0.png?1311)
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