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

sai roi pham anh quan

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

Mk thấy 2 bạn kia hình như bạn nào cũng đúng thì phải!
NHƯNG MK SẼ THÍCH BẠN NÀO LÀM GỌN NHANH HƠN!

     \(1990\times1990-1992\times1988\)

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

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

  \(=2\times1990-2\times1988\)

  \(=2\times\left(1990-1988\right)\)

  \(=2\times2\)

   \(=4\)

free fire

liên quân

yamatecuraxai

a) 1990.1990 - 1992.1988

= 1990.1990 - (1990 + 2)(1990 - 2)

= 1990.1990 - 1990.1990 + 4 = 4

b) (1374.57 + 687.86) : (870 + 235.122)

= (1374.57 + 687.2.43) : (10.87 + 5.47.2.61)

= (1374.57 + 1374.43) : (10.87 + 10.47.61)

= [1374(57 + 43)] : [10(87 + 47.61)]

= (1374.100) : [10(87 + 47.61)] (đoạn này mẫu số xấu quá, hetcuu)

c) (124.237 + 152) : (870 + 235.122)

= (124.235 + 124.2 + 152) : (870 + 235.122)

= (122.235 + 2.235 + 2.124 + 152) : (870 + 235.122)

= (122.235 + 470 + 248 + 152) : (235.122 + 870)

= (122.235 + 870) : (235.122 + 870) = 1

d) \(\dfrac{423134\cdot846267-423133}{846267\cdot423133+423134}=\dfrac{423133\cdot846267+846267-423133}{846267\cdot423133+423134}\)

\(=\dfrac{423133\cdot846267+423134}{846267\cdot423133+423134}=1\)

1990x1990-1992x1988

=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