2xC=2+6+12+20+...+2450

2xC=1x2+2x3+3x4+4x5+...+49x50

6xC=1x2x3+2x3x3+3x4x3+...+49x50x3

6xC=1x2x3+2x3x(4-1)+3x4x(5-2)+...+49x50x(51-48)

6xC=1x2x3+2x3x4-1x2x3+....+49x50x51-48x49x50

6xC=49x50x51

6xC=124950

C=20825

k nhé đúng 100%

2C = 2+6+12+20+30+...+2450

2C = 1.2+2.3+3.4+4.5+5.6+...+49.50

3C = 1.2.3+2.3.3+3.4.3+4.5.3+5.6.3+...+49.50.3

3C = 1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+5.6.(7-4)+...+49.50.(51-48)

3C = 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+5.6.7-4.5.6+5.6.7-4.5.6+....+49.50.51-48.49.50

=>C=49.50.51 : 3 =>C=4650

2xC=2+6+12+20+...+2450

2xC=1x2+2x3+3x4+4x5+...+49x50

6xC=1x2x3+2x3x3+3x4x3+...+49x50x3

6xC=1x2x3+2x3x(4-1)+3x4x(5-2)+...+49x50x(51-48)

6xC=1x2x3+2x3x4-1x2x3+....+49x50x51-48x49x50

6xC=49x50x51

6xC=124950

C=20825

cho mình 1 like nhé

Mình cảm ơn nhe!

số các số hạng là:(1225-1):+1=613 (số hạng)

tổng C =(1225+1)x 613:2=375769

\(=\dfrac{1\cdot2+2\cdot3+3\cdot4+4\cdot5+...+49\cdot50}{2}\)

\(=\dfrac{49\cdot50\cdot51}{3}=49\cdot50\cdot17=41650\)

\(2C=2+6+12+20+...+2450\)

\(2C=1.2+2.3+3.4+4.5+...+49.50\)

\(6C=1.2.3+2.3.3+3.4.3+...+49.50.3\)\(6C=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+49.50.\left(51-48\right)\)

\(6C=1.2.3+2.3.4-1.2.3+...+49.50.51-49.50.51\)

\(6C=49.50.51\)

\(6C=124950\)

\(C=20825\)

cảm ơn bn 

A = -1 - 1/3 - 1/6 - 1/10 - 1/15 - ... - 1/1225

A = -(1 + 1/3 + 1/6 + 1/10 + 1/15 + ... + 1/1225)

A = -(2/2 + 2/6 + 2/12 + 2/20 + 2/30 + ... + 2/2450)

A = -2.(1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + ... + 1/49.50)

A = -2.(1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + ... + 1/49 - 1/50)

A = -2.(1 - 1/50)

A = -2.49/50

A = -49/25

a: \(A=\left(\frac{136}{15}-\frac{28}{5}+\frac{62}{10}\right)\cdot\frac{21}{24}\)

\(=\left(\frac{272}{30}-\frac{168}{30}+\frac{186}{30}\right)\cdot\frac78\)

\(=\frac{290}{30}\cdot\frac78=\frac{29}{3}\cdot\frac78=\frac{203}{24}\)

b: \(B=\frac56+6\frac56\left(11\frac{5}{20}-9\frac14\right):8\frac13\)

\(=\frac56+\frac{41}{6}\cdot\left(11+\frac14-9-\frac14\right):\frac{25}{3}=\frac56+\frac{41}{6}\cdot2\cdot\frac{3}{25}\)

\(=\frac56+\frac{41}{25}=\frac{125}{150}+\frac{246}{150}=\frac{371}{150}\)

c: \(C=1+3+6+\cdots+1225\)

\(=\frac12\left(2+6+12+\cdots+2450\right)=\frac12\cdot\left(1\cdot2+2\cdot3+\cdots+49\cdot50\right)\)

\(=\frac12\cdot\left\lbrack1\left(1+1\right)+2\left(2+1\right)+\cdots+49\left(49+1\right)\right\rbrack=\frac12\cdot\left\lbrack\left(1+2+\cdots+49\right)+\left(1^2+2^2+\cdots+49^2\right)\right\rbrack\)

\(=\frac12\cdot\left\lbrack49\cdot\frac{50}{2}+\frac{49\left(49+1\right)\left(2\cdot49+1\right)}{6}\right\rbrack\)

\(=\frac12\cdot\left\lbrack49\cdot25+49\cdot50\cdot\frac{99}{6}\right\rbrack=\frac12\cdot\left\lbrack49\cdot25+49\cdot25\cdot33\right\rbrack=\frac12\cdot49\cdot25\cdot\left(33+1\right)\)

\(=49\cdot25\cdot\frac{34}{2}=49\cdot25\cdot17=20825\)