dung xich ma

\(\frac{149.150.151}{3}-\frac{2.3.4}{3}=1124942\)

Ta có :  B = 4.5+ 5.6 + 6.7 + .... + 49.50

=> 3B = 3.4.5 + 5.6.3 + 6.7.3 + ... + 49.50.3

=> 3B = 4.5.(6 - 3) + 5.6.(7 - 4) + 6.7.(8 - 5) + ... + 49.50.(51 - 48)

=> 3B = 4.5.6 - 3.4.5 + 5.6.7 - 4.5.6 + 6.7.8 - 5.6.7 + ... + 49.50.51 - 48.49.50

=> 3B = 49.50.51 - 3.4.5

=> 3B =  124890

=> B = 41630

Vậy B = 41630

\(A=\dfrac{3}{4\cdot5}+\dfrac{3}{5\cdot6}+...+\dfrac{3}{59\cdot60}\\ =3\left(\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+...+\dfrac{1}{59\cdot60}\right)\\ =3\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{59}-\dfrac{1}{60}\right)\\ =3\left(\dfrac{1}{4}-\dfrac{1}{60}\right)=3\left(\dfrac{15}{60}-\dfrac{1}{60}\right)\\ =3\cdot\dfrac{7}{30}=\dfrac{7}{10}\)

=(99.100.101-0.1.2):3=333300

Đặt A= 1.2+2.3 +.......+99.100

3A= 1.2.3+2.3.4+3.4.3 +......+ 99.100.3

3A= 1.2. (3 - 0) + 2.3.(4 - 1) +3.4. (5 - 2)....... . 99.100. (101 - 98)

3A = (1.2.3 + 2.3.4 + 3.4.5 +...... + 99.100.101) - (0.1.2 + 1.2.3 + 2.3.4 +.......+ 98.99.100)

3A = 99.100.101 - 0.1.2

3A = 999900 - 0

3A= 999900

A= 999900 : 3

A = 333300

Học tốt nha!

Đặt A = 1.2 + 2.3 + ... + 99.100

=> 3A = 1.2.3 + 2.3.(4-1) + ... + 99.100.(101-98)

=> 3A = 1.2.3 + 2.3.4 - 1.2.3 + ... + 99.100.101 - 98.99.100

=> 3A = 99.100.101

=> A = 333300

what

1/4*5+1/5*6+1/6*7+...+1/39*40

=1/4-1/5+1/5-1/6+1/6-1/7+...+1/39-1/40

=1/4-1/40

=9/40

thoyy,tui xin chịu ạ

=1/4+1/5-1/5+...-1/39+1/40

=1/4+1/19

=.....(tự tính nka)

\(\dfrac{1}{4.5}+\dfrac{1}{5.6}+...+\dfrac{1}{39.40}\)

\(=\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{39}-\dfrac{1}{40}\)

\(=\dfrac{1}{4}-\dfrac{1}{40}\)

\(=\dfrac{9}{40}\)

140.28

bạn giải ra đi

3s1=(2.3+3.4+4.5+5.6+...+15.16+16.17).3

3s1=2.3.3+3.4.3+4.5.3+5.6.3+....+15.16.3+16.17.3

3s1=2.3(4-1)+3.4(5-2)+4.5(6-3)+5.6(7-4)+...+15.16(17-14)+16.17(18-15)

3s1=2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+15.16.17-14.15.16+16.17.18-15.16.17

3s1=(16.17.18-1.2.3)/3

s1=1630