2/(11x13) = 1/11 - 1/13 ; 2/(13x15) = 1/13 - 1/15 ; 2/(15x17) = 1/15 - 1/17 ; 2/(17x19) = 1/17 - 1/19 ; 2/(19x21) = 1/19 - 1/21

2/(11x13) + 2/(13x15) + 2/(15x17) + 2/(17x19) 2/(19x21) =

1/11 - 1/13 + 1/13 - 1/15 + 1/15 - 1/17 + 1/17 - 1/19 + 1/19 - 1/21 = 1/11 - 1/21 = 101231

Hay:   10/231 x 462 - X = 19

20 - X = 19

X = 20 - 19

X = 1,đó là phần b

3/5x+2/3x=-76 <=> 19/15x=-76 <=> x=-60,đó là phần a

Bài 1: 

\(101\cdot125+101\cdot25-101\cdot50\)

\(=101\cdot\left(125+25-50\right)\)

\(=101\cdot100\)

\(=10100\)

Bài 2: 

\(76\cdot115+56\cdot24+59\cdot24\)
\(=76\cdot115+24\cdot\left(56+59\right)\)

\(=76\cdot115+24\cdot115\)

\(=115\cdot\left(76+24\right)\)

\(=115\cdot100\)

\(=11500\)

còn bài 3,4,5 thì sao ak 

1/3x + 3/4x - 75% = -5 1/4

=> x - 3/4 = -21/4

=> x = -21/4 + 3/4

=> x = -9/2

60%x + 5/6x = -76

=> 3/5x + 5/6x = -76

=> 43/30x = -76

=> x = -76 : 43/30

=> x = -2280/43

\(\frac{1}{3}x+\frac{3}{4}x-75\%=-5\frac{1}{4}\)

\(\frac{13}{12}x-\frac{3}{4}=-\frac{21}{4}\)

\(\frac{13}{12}x=-\frac{9}{2}\)

\(x=-\frac{54}{13}\)

\(60\%.x+\frac{2}{3}x=-76\)

\(\frac{3}{5}x+\frac{2}{3}x=-76\)

\(\frac{19}{15}x=-76\)

\(x=-60\)

Bài 1:

a. $=(-25)(-4)(-35)=100(-35)=-3500$

b. $=16-10=6$

c. $=180-(-16)-(-36)=180+16+36=232$

d. $=250-200:[1(-3)^2+(-8)]$

$=250-200:(9-8)=250-200=50$

2.

$60+2(12-x)=-48$

$2(12-x)=60-(-48)=60+48=108$

$12-x=108:2=54$

$x=12-54=-42$
 

Bài 1:

a. $=(-25)(-4)(-35)=100(-35)=-3500$

b. $=16-10=6$

c. $180-(-16)-(-36)=180+16+36=196+36=232$

d. $=250-200:[2000.(-3).2-6]$

$=250-200:[2000.(-6)+(-6)]$

$=250-200:[(-6)(2000+1)]=250-200[(-6).2001]$

$=250+200.6.2001=250+2401200=2401450$

Bài 2:

$60+2(12-x)=-48$

$2(12-x)=-48-60=-108$
$12-x=-108:2=-54$

$x=12-(-54)=66$

\(3\left(x+2\right)+2x=76\)

\(3x+6+2x=76\)

\(3x+2x+6=76\)

\(5x=76-6\)

\(\Rightarrow5x=70\)

\(\Rightarrow x=70:5\)

\(\Rightarrow x=14\)

a, 55 - x + 33 = 76

=> - x = 76 - 55 - 33

=> - x = - 12

=> x = 12

vậy_

b, (x + 1) + (x + 2) + (x + 3) +.......+ (x + 100) = 5550

=> x + 1 + x + 2 + x + 3 + ... + x + 100 = 5550

=> (x + x + x + ... + x) + (1 + 2 + 3 + ... + 100) = 5550

=> 100x + 5050 = 5550

=> 100x = 500

=> x = 5

vậy_

a,x=12