= 1/1x2+1/2x3+...+1/7x8

= 1-1/2+1/2-...-1/8

= 1-1/8

=7/8

a/ \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}=\)

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}=\)

= 1-1/8 = 7/8

b/ = (17/9+1/9)+(19/13+7/13)+(14/6+10/6) = 18/9 + 26/13 + 24/6 = 2+2+4 = 8

\(a)\) \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}\)

\(=\)\(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+\frac{1}{7.8}\)

\(=\)\(\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}\)

\(=\)\(1-\frac{1}{8}\)

\(=\)\(\frac{7}{8}\)

Áp dụng công thức \(\frac{a}{b\left(b+a\right)}=\frac{1}{b}-\frac{1}{b+a}\) \(\left(a,b>0\right)\) nhé 

Chúc bạn học tốt ~ 

1+1+2+2+3+3+4+4+5+5+6+6+7+7+8+9+8+9+10+12+13+14+15+16+17+18+19+20+30+1000000=10000274

10000274

10000274 nha

1+2+3+4+5+6+7+8+9+10=55

11+12+13+14+15+16+17+18+19+20=155

1+2+3+4+5+6+7+8+9+10+11+12+13+14 +15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30-50-53=362

1+2+3+4+5+6+7+8+9+10=55

11+12+13+14+15+16+17+18+19+20=155

1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30-50-53=362

a. ( 23 - 21) + ( 19 - 17) + ( 15 - 13) + ( 11 - 9) + ( 7 - 5) + ( 3 - 1) 

= 2 + 2 + 2 + 2 + 2 + 2 

= 2 x 6

= 12

b. ( 24 - 22 ) + ( 20 - 18 ) + ( 16 - 14 ) + ( 12 - 10) + ( 8 - 6 ) + ( 4 - 2) 

= 2 + 2 + 2 + 2 + 2 + 2 

= 2 x 6 

= 12

a)=(3/8+10/16)+(7/12+10/24)

=1+1=2

c)=(4/6+14/6)+(7/13+19/13)+(17/9+1/9)

=3+2+2=7

\(1+2+3+4+5+6+7+8+9+10=55\)

\(11+12+13+14+15+16+17+18+19+20=155\)

=55

=155

=438

Tích cho mình đi

a; A = -1 + 3 - 5 + 7 - 9 + 11 - 13 + 15 - 17

   A = (-1 + 11) + ( 3 - 13) + (-5 + 15) + (7 - 17) - 9

   A = 10 - 10 + 10 - 10 - 9

 A = (10 - 10) + (10 - 10) - 9

 A = 0 + 0 - 9

A = -9 

b; B = 1+2-3-4+5+6-7-8+9+10-11-12+13+14-15-16+17+18-19-20

B = (1+2-3-4) + (5+6-7-8)+(9+10-11-12)+(13+14-15-16)+(17+18-19-20)

B= -4+(-4)+(-4)+(-4)+(-4) 

B= -4 . 5

B= -20

c= - 1/10 nha

Áp dụng công thức tính dãy số cách đều ta có: 

1 + 2 + 3 + ... + 20

= (1 + 20) x 20 : 2

= 21 x 10

= 210

Cách 1:

Áp dụng công thức tính dãy số ta có

[(20 - 1) : 1 + 1]  x  (20 + 1) : 2     

= 20 x 21 : 2

= 10 x 21

= 210 

Cách 2 :

1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13 + 14 + 15 + 16 + 17 + 18 + 19 + 20

= (20 +1 ) + (19 + 2) +....+ (11+ 10)

= 21 + 21 + ....+ 21  ( 10 số 21)

= 21 x 10 

= 210

( 20 + 1 ) . 20 : 2 = 210