47.8 - 27.9 + 47.12 - 27.11 = 47. (8 +12) - 27.(9 +11) = (47-27).20 = 400

400 nha

91.25 - 91.13 - 91.12 = 91.(25 - 13 - 12) = 0

Bài 3:

a: Ta có: \(23\left(42-x\right)=23\)

\(\Leftrightarrow42-x=1\)

hay x=41

b: Ta có: 15(x-3)=30

nên x-3=2

hay x=5

Bài 1: 

a: 32+89+68=100+89=189

b: 64+112+236=300+112=412

c: \(1350+360+650+40=2000+400=2400\)

a) \(125\times25\times8\times4\)
\(\Rightarrow\left(125\times8\right)\times\left(25\times4\right)\)
\(\Rightarrow1000\times100\)
\(\Rightarrow100000\)

\(\text{a. 125 x 25 x 8 x 4 }\\ =125.8.25.4\\ =1000.100=100000\)

\(\text{ b. 25 x 32 x 125 x 15}\\ =25.15.32.125\\ =375.4000=1500000\)

b.25 x 32 x 125 x 15=25 x 4 x 8 x 125 x 15

                                =(25 x 4) x (125 x 8) x15

                                =100 x 1000 x 15 = 1500000

Hơi dài nên mình ko làm 

a) 35.(x-10)=35

         x-10= 35:35

         x-10= 1

             x= 1+10

            x= 10

b) 21.(32-x) =21

          32-x = 21:21

          32-x = 1

               x= 32-1

                x= 31

a)\(35.\left(x-10\right)=35\Rightarrow x-10=35:35=1\Rightarrow x=10+1=11\)

b) \(21.\left(32-x\right)=21\Rightarrow32-x=21:21=1\Rightarrow x=32-1=31\)

c) \(25.\left(2x.4\right).12=0\Rightarrow25.\left(2x.4\right)=0:12=0\Rightarrow2x.4=0:25=0\)

\(\Rightarrow2x=0:4=0\Rightarrow x=0:2=0\)

a) \(82\cdot78\)

\(=\left(80+2\right)\cdot\left(80-2\right)\)

\(=80^2-2^2\)

\(=6400-4\)

\(=6396\)

b) \(87\cdot93\)

\(=\left(90-3\right)\left(90+3\right)\)

\(=90^2-3^2\)

\(=8100-9\)

\(=8091\)

c) \(125^2-25^2\)

\(=\left(125-25\right)\left(125+25\right)\)

\(=100\cdot150\)

\(=15000\)

375 : 25 - 125 : 25 = (375 - 125) : 25 = 10

a,=235.(64+36)=235.100=23500

b,=376.(236-136)=376.100=37600

c,=459.(99+1)=459.100=45900

d,=567.(125-25)=567.100=56700

\(=235\times\left(64+36\right)=235\times100=23500\)

\(=376\times\left(236-136\right)=376\times100=37600\)

\(=459\times\left(99+1\right)=459\times100=45900\)

\(=567\times\left(125-25\right)=567\times100=56700\)

`235 xx 64 + 235 xx 36`

`= 235 xx 100`

`= 23500`

________________

`376 xx 236 - 136 -136 xx 376`

`= 376 xx (236 - 136)`

`= 376 xx 100`

`= 37600`

_________________

`459 xx 99 + 459 xx 1`

`= 459 xx (99 + 1)`

`= 459 xx 100`

`= 45900`

________________

`567 xx 125 - 25 xx 167`

`= 567 xx (125 - 25)`

`= 567 xx 100`

`= 56700`

a, 32.(-64) - 64.68

   = -64( 32 + 68)

= -64 .100

= - 6400

b, -54.76 + 46.(-76)

= -76.(54 + 46)

= - 76. 100

= - 7600

c, 75.18 + 18.25

= 18.(75 + 25)

= 18 . 100

= 1800

d,  (-4).(3).(-125)(-25).(-8)

=   3.(-4. (-25).((-125).(-8)

= 3.100.1000

= 300000