ta có : 1/5+1/15=1-1/15=14/15

vậy 1/5+1/15+1/45+1/135+1/405=1-1/405=404/405

404/405 NHỚ **** ĐÓ

\(\frac{121}{405}\)

ta có :1/5 + 1/15 = 1- 1/15 =14/15 

vậy :1/5 + 1/15 + 1/45 + 1/135 + 1/405 = 1- 1/405 = 404/405

đáp số : 404/405

=>A=6/5-(19/7+26/5).

=>A=6/5-26/5-19/7.

=>A=-20/5-19/7.

=>A=-4+(-19/7)=-4 19/7 (đay là hỗn số).

B=47*45-47*15-47*45+47*15.

=>B=(47*45-47*45)+(47*15-47*15).

=>B=0+0.

=>B=0.

tk nha.

-45 x 65 - 45 x 30 - 45 x 3

= -45 x (65 + 30 + 3)

= -45 x 98

= -230

a) -5x (-9) = -11\(\Rightarrow-5\chi=\frac{11}{9}\Rightarrow\chi=\frac{11}{9}:\left(-5\right)\Rightarrow\chi=\frac{11}{9}.\frac{-1}{5}\Rightarrow\chi=\frac{-11}{45}\)b) 25 - (x + 5 ) = -415 - ( 15-415)\(\Rightarrow25-\left(\chi+5\right)=-415-15+415\)

\(\Rightarrow25-\left(\chi+5\right)=\left(-415+415\right)-15\)

\(\Rightarrow25-\left(\chi+5\right)=-15\Rightarrow\chi+5=25+15\)

\(\Rightarrow\chi+5=40\Rightarrow\chi=35\)

HTDT

1.

-45 x 65 - 45 x 30 - 45 x 3 = 45 (-65-30-3) = 45 x ( -98 ) = -4410

 2.

a) -5x (-9) = -11

45x = -11

x = -11 : 45

x = \(\frac{-11}{45}\)

b) 25 - (x + 5) = -415 - ( 15 - 415)

25 - x - 5 = -415 -15 + 415

(25 - 5) - x = (-415 + 415) - 15

20 - x = -15

x = 20 - (-15)

x = 20 + 15

x = 35

a)5.(-8).2.(-3)

=[5.(-2)].[(-8).(-3)]

=-10.24

=-240

b)(45-135+72)-(45+72)

=45-135+72-45-72

=(45-45)+(72-72)-135

=0+0-135

=-135

c)3.(-5)².2+2.(-5)-20

=3.25.2+2(-5)-20

=75.2+(-10)-20

=150+(-30)

=120

d)86.(-46)+46.27-46.41

=86.(-1).46+46.27-46.41

=-86.46+46.27-46.41

=46(-86+27-41)

=46(-100)

-4600

e)34(15-10)-15(34-10)

=34.15-34.10-15.34+15.10

=(34.15-15.34)+(15.10-34.10)

=[34(15-15)]+[10(15-34)]

=34.0+10(-9)

=0+(-90)

=-90

1 : Tính nhanh:

a) 2376 + 3425 - 376 - 425 = ( 2376 - 376 ) + ( 3425 - 425 )

                                       = 2000 + 3000

                                       = 5000

b)4638 - 2437 + 5362 - 7563 = 2210 + ( - 2210 )

                                          = 0

c) 45 + 45 + 45 + 45 + 15 + 15 + 15 + 15

  = 45 x 4 + 15 x 4

 = ( 45 + 15 ) x 4

 = 60 x 4

 = 240

d) 4 x 125 x 25 x 8 = ( 4 x 25 ) x ( 125 x 8 )

                            = 100 x 1000

                            = 100000

e) 2 x 8 x 50 x 25 x 125 = ( 2 x 50 ) x ( 8 x 125 ) x 25

                                   = 100 x 1000 x 25

                                   = 100000 x 25

                                   = 2500000

2 : Tính nhanh bằng 2 cách:

C1 : 125 + 125 + 125 + 125 - 25 - 25 - 25 - 25

    = ( 125 - 25 ) + ( 125 - 25) + ( 125 - 25 ) + ( 125 - 25 )

    =100 + 100 + 100 + 100 

    = 100 x 4 

    = 400

C2 : 125 + 125 + 125 + 125 - 25 - 25 - 25 - 25

     = 125 x 4 - 25 x 4

     = ( 125 - 25 ) x 4

     = 100 x 4

     = 400

1.

a. 5000
b. 0

c. 240

d. 100000

e. 2500000

2.

400

1: Tính nhanh:

a, 2376 + 3425 - 376 - 425

 = ( 2376 - 376 ) + ( 3425 - 425 )

 = 2000 + 3000

 = 5000.

b, 4638 - 2437 + 5362 - 7563

 = 2210 + (-2210)

 = 0.

c, 45 + 45 + 45 + 45 + 15 + 15 + 15 + 15 + 15

 = ( 45 + 15 ) +  ( 45 + 15 ) +  ( 45 + 15 ) +  ( 45 + 15 ) 

 = 60 + 60 + 60 + 60

 = 240.

d, 4 x 125 x 25 x 8 

 = ( 4 x 25 ) x ( 125 x 8 )

 = 100 x 1000

 = 100000.

e, 2 x 8 x 50 x 25 x 125

 = ( 2 x 50 ) x ( 8 x 125 ) x 25

 = 100 x 1000 x 25

 = 2500000.

2: Tính nhanh bằng hai cách:

  125 + 125 + 125 + 125 - 25 - 25 - 25- 25

= ( 125 - 25 ) + ( 125 - 25 ) + ( 125 - 25 ) + ( 125 -25 )

= 100 + 100 + 100 + 100

= 400.

1.

\(\left(572\cdot7+266\right)\cdot\left(366\cdot9-168\cdot18\right)\cdot\left(346\cdot6-348\right)\)

\(=\left(286\cdot7\cdot2+133\cdot2\right)\cdot\left(366\cdot9-168\cdot2\cdot9\right)\cdot\left(173\cdot6\cdot2-174\cdot2\right)\)

\(=\left(2002\cdot2+133\cdot2\right)\cdot\left(366\cdot9-336\cdot9\right)\cdot\left(1038\cdot2-174\cdot2\right)\)

\(=\left[2\cdot\left(2002+133\right)\right]\cdot\left[9\cdot\left(366-336\right)\right]\cdot\left[2\cdot\left(1038-174\right)\right]\)

\(=2\cdot2135\cdot9\cdot30\cdot2\cdot864\)

\(=4270\cdot9\cdot30\cdot2\cdot864\)

\(=\left(4270\cdot30\right)\cdot9\cdot2\cdot864\)

\(=\left(427\cdot10\right)\cdot\left(3\cdot10\right)\cdot9\cdot2\cdot864\)

\(=\left(427\cdot3\right)\cdot\left(10\cdot10\right)\cdot9\cdot2\cdot864\)

\(=1281\cdot100\cdot9\cdot2\cdot864\)

\(=\left(1281\cdot100\right)\cdot\left(9\cdot2\cdot864\right)\)

\(=\left(1281\cdot100\right)\cdot15552\)

\(=\left(1281\cdot15552\right)\cdot100\)

\(=19922112\cdot100\)

\(=1992211200\)

2.

\(\left(1+3+5+7+...+97+9\right)\cdot\left(45\cdot3-15\cdot2-45\right)\)

\(=\left(1+3+5+7+...+97+9\right)\cdot\left(15\cdot3\cdot3-15\cdot2-15\cdot3\right)\)

\(=\left(1+3+5+7+...+97+9\right)\cdot\left[15\cdot\left(3\cdot3\right)-15\cdot2-15\cdot3\right]\)

\(=\left(1+3+5+7+...+97+9\right)\cdot\left(15\cdot9-15\cdot2-15\cdot3\right)\)

\(=\left(1+3+5+7+...+97+9\right)\cdot\left[15\cdot\left(9-2-3\right)\right]\)

\(=\left(1+3+5+7+...+97+9\right)\cdot15\cdot4\)

\(=\left[\left(1+3+5+7+...+97\right)+9\right]\cdot\left(15\cdot4\right)\)

Trong \(\left(1+3+5+7+...+97\right)\) có số số hạng là:

        \(\left(97-1\right)\div2+1=49\) ( số hạng )

\(\Rightarrow\left[\left(1+3+5+7+...+97\right)+9\right]\cdot\left(15\cdot4\right)\)

\(=\left[\left(97+1\right)\cdot49\div2\right]\cdot\left(15\cdot4\right)\)

\(=2401\cdot60\)

\(=\left(2401\cdot6\right)\cdot10\)

\(=14406\cdot10\)

\(=144060\)

3.

\(\left(180\div15-132\div11\right)\cdot\left(57869-297\div11\cdot108\right)\)

\(=\left(12-12\right)\cdot\left(57869-297\div11\cdot108\right)\)

\(=0\cdot\left(57869-297\div11\cdot108\right)\)

\(=0\)

Sửa lại bài 2; dòng 11 ( từ đề bài bài 2 ):

\(=\left\{\left[\left(97+1\right)\cdot49\div2\right]+9\right\}\cdot\left(15\cdot4\right)\)

\(=\left(2401+9\right)\cdot\left(15\cdot4\right)\)
\(=2410\cdot\left(15\cdot4\right)\)

\(=2410\cdot60\)

\(=\left(241\cdot10\right)\cdot\left(6\cdot10\right)\)

\(=\left(241\cdot6\right)\cdot\left(10\cdot10\right)\)

\(=1446\cdot10\)

\(=14460\)

a: \(=\dfrac{-2}{12}-\dfrac{5}{12}+\dfrac{7}{12}=0\)

b: \(=\left(\dfrac{4}{45}-\dfrac{1}{45}+\dfrac{7}{45}+\dfrac{4}{45}-\dfrac{2}{45}-\dfrac{9}{45}\right)=\dfrac{3}{45}=\dfrac{1}{15}\)

\(\text{Đặt A }=\frac{1}{15}+\frac{2}{45}+\frac{3}{135}+\frac{4}{345}+\frac{5}{759}\)

\(\Rightarrow A=\frac{1}{3.5}+\frac{2}{5.9}+\frac{3}{9.15}+\frac{4}{15.23}+\frac{5}{23.33}\)

\(\Rightarrow2A=2\left(\frac{1}{3.5}+\frac{2}{5.9}+\frac{3}{9.15}+\frac{4}{15.23}+\frac{5}{23.33}\right)\)

\(\Leftrightarrow2A=\frac{2}{3.5}+\frac{4}{5.9}+\frac{6}{9.15}+\frac{8}{15.23}+\frac{10}{23.33}\)

\(\Leftrightarrow2A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{15}+\frac{1}{15}-\frac{1}{23}+\frac{1}{23}-\frac{1}{33}\)(Tại sao ra vậy cứ ib giải thích cho)

\(\Rightarrow2A=\frac{1}{3}-\frac{1}{33}\)

\(\Leftrightarrow2A=\frac{10}{33}\)

\(\Rightarrow A=\frac{10}{33}:2=\frac{5}{33}\)

Chúc bạn học tốt !!!

Cảm ơn bạn nhìu mink cũng chúc bạn hok tốt nhé ! ^^

Đặt A =115+245+3135+4345+5759Đặt A =115+245+3135+4345+5759

⇒A=13.5+25.9+39.15+415.23+523.33⇒A=13.5+25.9+39.15+415.23+523.33

⇒2A=2(13.5+25.9+39.15+415.23+523.33)⇒2A=2(13.5+25.9+39.15+415.23+523.33)

⇔2A=23.5+45.9+69.15+815.23+1023.33⇔2A=23.5+45.9+69.15+815.23+1023.33

⇔2A=13−15+15−19+19−115+115−123+123−133⇔2A=13−15+15−19+19−115+115−123+123−133(Tại sao ra vậy cứ ib giải thích cho)

⇒2A=13−133⇒2A=13−133

⇔2A=1033⇔2A=1033

⇒A=1033:2=533⇒A=1033:2=533

Chúc bạn học tốt !!!

a,15*(4+5)

c1

15*(4+5)

=15x9

=135

c2

15*(4+5)

=15x4+15x5

=60+75

=135

#hoktot

#hungb

a) 15 x (4 + 5)

= 15 x 9

= 135

15 x (4 + 5)

= 15 x 4 + 15 x 5

= 60 + 75

= 135

b) 45 x (8 - 5)

= 45 x 3

= 135

45 x (8 - 5)

= 45 x 8 - 45 x 5

= 360 - 225

= 135

còn phần b.