65x42-130x21+1998

=65x (2x21)-130x21+1998

=(65x2)x21-130x21+1998

=130x21-130x21+1998

=0+1998

=1998

\(\frac{2011\times2012+2013\times21+1991}{2012\times2013-2012\times2012}\)

\(=\frac{2011\times2012+2013\times\left(21+1991\right)}{2012\times2013-2012\times2012}\)

\(=\frac{2011\times2012+2013\times2012}{2012\times2013-2012\times2012}=\frac{2011}{2012}\)

Bạn dùng tính chất phân phối, giao hoán để làm nhé

A+0   B=2400   c=420

1) a) 53 x 39 + 47 x 39 - 53 x 21 - 47 x 21

= 39 x (53 + 47) - 21 x (53 - 47)

= 39 x 100 - 21 x 6

= 3900 - 126

= 3774

b) 2 x 53 x 12 + 4 x 6 x 87 - 3 x 8 x 40

= 1272 + 2088 - 960

= 3360 - 960

= 2400

c) 5 x 7 x 77 - 7 x 60 + 47 x 25 - 15 x 42

= 2695 - 420 + 1175 - 630

= 2275 + 545

= 2820

2) a) x : [(1800 + 600) : 30] = 560 : (315 - 35)

                     x : (2400 : 30) = 560 : 280

                                  x : 80 = 2

                                         x = 2 x 80

                                    => x = 160

b) [(250 - 25) : 15] : x = (450 - 60) : 130

                225 : 15 : x = 390 : 130

                         15 : x = 3

                               x  = 15 : 3

                           => x = 5

Học tốt!!!

20,16 x 49 + 20,16 x 50 + 20,16

= 20,16 x (49+50+1)

= 20,16 x 100

=2016

\(=\dfrac{21}{11}\cdot\dfrac{22}{17}\cdot\dfrac{34}{42}=\dfrac{1}{2}\cdot2\cdot2=2\)

21/11 x 22/17 : 42/34

=     41/17  : 42/34

=             41/21

a, 35x 34+35x 38 + 65x 75+ 65x 45=10437

b, 36x 28 + 36x 82 + 64x 69 + 64 x41=11000

c, 254x 399 - 145/ 254+ 399 x 253=202292,4291

\(21\times35+42\times31+63\)

\(=21\left(35+31\times2+3\right)\)

\(=21\times100\)

\(=2100\)

a) \(21\times35+42\times31+63\)

\(=21\times35+21\times2\times31+21\times3\)

\(=21\times35+21\times62+21\times3\)

\(=21\times\left(35+62+3\right)\)

\(=21\times100\)

\(=2100\)

_Chúc bạn học tốt_

Trả lời:

\(21\cdot35+42\cdot31+63\)

\(=21\cdot\left(35+31\cdot2+3\right)\)

\(=21\cdot100\)

\(=2100\)

Hc tốt #

b,21/17x13/14x56x3/42

=21/17x3/42x56x13/14

=21x3/17x42 x 56x13/14

=21x3/17x21x2 x 14x4x13/14

=3/17x2 x 4x13

=3/24 x 52=3x52/24

=3x52/3x8

=52/8=13/2

a, 6/7 x 16/15 x 7/6 x 21/32

=(6/7 x 7/6)x16/15x21/32

=16/15x21/32

=16x21/15x32

=16x3x7/3x5x16x2

=7/5x2=7/10