a) 7.8.39+56.30+14.4.31

= 56.39+56.30+56.31

= 56.(39+30+31)

= 56.100

=5600

b) 175+35-275+100

= (175-275)+100+35

= -100+100+35

= 0+35

=35

c) 350.7+140.7+7:7

= 7.(350+140)+1

= 7.490+1

= 3430+1

=3431

a, 7.8.39+56.30+14.4.31

=56.39+56.30+56.31

=56.(39+30+31)

=56.100

=5600

b, 175+35-275+100

=(175+100-275)+35

=(375-375)+35

=0+35

=35

c, 350.7+140.7+7:7

=7.(350+140)+7:7

=7.490+1

=3430+1

=3431

a, 5600

b, 35

c, 3431

54 x 275 + 825 x 15 + 275 

=14850 + 825 x 15 + 275 

=14850 + 12375+275

=14850 + 12650

=27500

477x1998-1998x427

=1998x(477-427)

=1998x50

=99900

1985:2+15:2

=(1985+15):2

=2000:2

=1000

40x113x25-20x112x50

=(40x25)x113-(20x50)x112

=1000x113-1000-112

=1000x(113-112)

=1000x1

=1000

câu 1, ??????????????

câu 2, [477-427], +[1998-1998]=50+0=50

câu 3,[1985+15]:2=2000:2=1000

câu 4, ??????????

chú thích ? là ko có cách tính nhanh

54 x 275 + 825 x 15 + 275 

=14850 + 825 x 15 + 275 

=14850 + 12375+275

=14850 + 12650

=27500

477x1998-1998x427

=1998x(477-427)

=1998x50

=99900

1985:2+15:2

=(1985+15):2

=2000:2

=1000

40x113x25-20x112x50

=(40x25)x113-(20x50)x112

=1000x113-1000-112

=1000x(113-112)

=1000x1

=1000

= 54x275+275x(3x15)+275

= 54x275+ 275x45+275

= 275x(54+45+1)

= 275x100

= 27500

27500

54 x 275 + 825 x 15 + 275

= 54 x 275 + 275 x 45 + 275

= 275 x ( 54 + 45 + 1 )

= 275 x 100

= 27500

c, 50+20+1=71

a)54x275+825x15+275                        b)(15+17+19+...+57+59)x(n:1-nx1)       

= 54x275+275x3x15+275                      =  A   x   (n-n)

= 54x275+275x45+275                           = A  x 0

= 275x(54+45+1)                                    = 0

= 275x100

=27500 

Cảm ơn rất nhiều vì mk cũng có câu hỏi giống nguyễn huyền trang

 54x72+54x27+54

=54x72+54x27+54x1

=54x(72+27+1)

=54x100

=5400

Hok tốt!

   54 * 72 + 54 * 27 + 54

= 54 * 72 + 54 * 27 + 54 * 1

= 54 * ( 72 + 27 + 1 )

= 54 * 100

= 5400.

54 × 72 + 54 x 27 + 54

= 54 x 72 + 54 x 27 + 54 x 1

= 54 x ( 72 + 27 + 1 )

= 54 x 100 

= 5400

54x342-44x342

=342x(54-44)

=342x10

=3 420

54x342-44x342

=342x(54-44)

=342x10

=3 420

54x342-44x342

=342x(54-44)

=342x10

=3 420

a,     54 x 275 + 825 x 15 + 275 = 54 x 275 + 3 x 275 x 15 + 275

                                              = 275( 54 + 3 x 15 + 1 )

                                             = 275 x 100

                                              = 27500

b , 19 x 29 x 39 x 49 x 59 x 69 x 79 x 89 x 99    có tận cùng là 9

=( 7*5*39)/(13*14*15)

=(7*5*3*13)/(13*7*2*5*3)

=1/2

 \(\frac{7}{13}x\frac{5}{14}x\frac{39}{15}\)

\(=\frac{7x5x39}{13x14x15}\)

\(=\frac{7x5x13x3}{13x7x2x5x3}\)

\(=\frac{1}{2}\)