\(C=\frac{1.3+1}{1.3}+\frac{2.4+1}{2.4}+\frac{3.5+1}{3.5}+...+\frac{47.49+1}{47.49}+\frac{48.50+1}{48.50}\)

\(=1+\frac{1}{1.3}+1+\frac{1}{2.4}+1+\frac{1}{3.5}+...+1+\frac{1}{47.49}+1+\frac{1}{48.50}\)

\(=48+1-\frac{1}{3}+\frac{1}{2}-\frac{1}{4}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{47}-\frac{1}{49}+\frac{1}{48}-\frac{1}{50}\)

\(=48+1+\frac{1}{2}-\frac{1}{49}-\frac{1}{50}=...\)

\(C=\frac{4}{1.3}+\frac{9}{2.4}+\frac{16}{3.5}+...+\frac{2401}{48.50}\)

\(C=\frac{2.2}{1.3}+\frac{3.3}{2.4}+\frac{4.4}{3.5}+...+\frac{49.49}{48.50}\)

\(C=\frac{2.3.4...49}{1.2.3...48}+\frac{2.3.4...49}{3.4.5...50}=49+\frac{2}{50}=\frac{49}{25}\)

thankyou mn rất nhìu

S = 1.3+2.4+3.5+ .....+48.50+49.51 

S = 1(2 + 1) + 2(3 + 1) + 3(4 + 1) + ... + 48(49 + 1) + 49(50 + 1)

S =1 + 1.2 + 2.3 + 2 + 3.4 + 3 + ... + 48.49 + 48 + 49.50 + 49

S = (1 + 2 + 3 + ... + 49) + (1.2 + 2.3 + 3.4 + ... + 49.50)

đặt A  = 1 + 2 + 3 + ... + 49

A = (49 + 1).49 : 2

A = 1225

đătj B = 1.2 + 2.3 + 3.4 + ... + 49.50

3B = 1.2.3 + 2.3.3 + 3.4.3 + ... + 49.50.3

3B = 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 49.50.(51 - 48)

3B = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 49.50.51 - 48.49.50

3B = 49.50.51

B = 41650

=> S = A + B = 41650 + 1225 = 42875

cảm ơn bạn nha Tomoe !!!

42875

Cho xin cách lam

1.3+2.4+ 3.5 +.........+ 48.50 + 49.51 =  [ 1+2+3+...............+49  ] x  [ 3+4+5.............+51 ]  =   1225  x 1323 =1620675

b: 6B=2*4*6+4*6*6+6*8*6+...+46*48*6+48*50*6

=2*4*6-2*4*6+4*6*8-4*6*8+...-44*46*48+46*48*50-46*48*50+48*50*52

=48*50*52

=>B=20800

d: 9D=1*4*9+4*7*9+...+46*49*9

=1*4*2+1*4*7-1*4*7+1*7*10-1*7*10+...+46*49*52-46*49*43

=1*2*4+46*49*52

=117216

=>D=13024

a: