99 + 87 + 99 + 13 + 99 + 45

= ( 99 + 99 + 99 ) + ( 87 + 13 ) + 45

= 297 + 100 + 45

= 397 + 45

= 442

99+87+99+13+99+45
=(99+99+99)+(87+13)+45
=297+100+45
=442

-1300

-3600

1544

13.(-87)+(-13).13= 13.(-87+(-13))

                        = 13.(-100)

                        =-1300

\(\dfrac{x-13}{87}+\dfrac{x-27}{73}+\dfrac{x-67}{83}+\dfrac{x-73}{27}=4\)

<=>\(\dfrac{x-13}{87}-1+\dfrac{x-27}{73}-1+\dfrac{x-67}{83}-1+\dfrac{x-73}{27}-1=0\)

<=>\(\dfrac{x-100}{87}+\dfrac{x-100}{73}+\dfrac{x-100}{83}+\dfrac{x-100}{27}=0\)

<=>\(\left(x-100\right)\left(\dfrac{1}{87}+\dfrac{1}{73}+\dfrac{1}{83}+\dfrac{1}{27}\right)=0\)

Do \(\dfrac{1}{87}+\dfrac{1}{73}+\dfrac{1}{83}+\dfrac{1}{27}>0\)

=>x-100=0

<=>x=100

sửa chỗ :\(\dfrac{x-67}{33}\)

1. 336 : x = 12

x = 336 : 12

x = 28

2. (x + 7) - 25 = 13

x + 7 = 13 + 25

x + 7 = 38

x = 38 - 7

x = 31

3. 87 - (73 - x) = 20

73 - x = 87 - 20

73 - x = 67

x = 73 - 67

x = 6

4. x - 105 : 21 = 15

x - 5 = 15

x = 15 + 5

x = 20

5. (x - 36) : 18 = 12

x - 36 = 216

x = 216 + 36

x = 252

a)45.(13+78)+9.(87+22).5

=45.91+(9.5).109

=45.91+45.109

=45.(91+109)

=45.200

=9000

b) 16.(27+75)+8.(53+25).2

=16.102+(8.2).78

=16.102+16.78

=16.(102+78)

=16.180

=2880

a) \((-15)+(-25) = -40\)

b) \((-87) + 13 = -74\)

(x-13)/87+(x-27)/73+(x-67)/33+(x-73)/27=4

=>(x-13-87)/87+(x-27-73)/73+(x-67-33)/33+(x-73-27)/27=4-1-1-1-1

=>(x-100)/87+(x-100)/73+(x-100)/33+(x-100)/27=0

=>(x-100)*(1/87+1/73+1/33+1/27)=0

=>x-100=0

=>x=100

\(\frac{7}{8}+\frac{18}{10}=\frac{35}{40}+\frac{72}{40}=\frac{107}{40}\)

\(\frac{123}{15}-\frac{85}{13}=\frac{1599}{195}-\frac{1275}{195}=\frac{324}{195}=\frac{108}{65}\)

\(\frac{78}{12}\times\frac{87}{89}=\frac{6786}{1068}=\frac{2262}{356}\)

5: \(=42\cdot\left(15+96+25+4\right)=42\cdot140=5880\)

6: \(=45\left(13+78+87+22\right)=45\cdot200=9000\)