A.nhân 2 lên

B.câu A nhân 2

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mk ko hiẻu

\(=\left(1+3+5+...+99+101\right)-\left(2+4+6+...98+100\right)\)
Thấy từ 1 đến 100 có (101-1)/2+1=51
=> 1+3+5+....+99+100=(1+101)x50/2=2601
Từ 2 đến 100 có (102-2)/2+1=50
=> 2+4+...+98+100=(2+100)X50/2=2550
=> D=2601-2550=51

2/2*4 + 2/4*6 + 3/6*8 + ... + 2/38*50

= 1/2 - 1/4 + 1/4 - 1/6 + 1/6 - 1/8 + .... + 1/38 - 1/50

= 1/2 - 1/50

= 24/50

=  12/25

Mk có cách khác câu d nè

\(D=\left(1-2\right)+\left(3-4\right)+...+\left(99-100\right)+101\)

     \(=-1-1-1-...-1+101\)(có 50 số -1)

     \(=-1\times50+101\)

      \(=51\)

chúc bn hok tốt

Đặt \(A=2.4+4.6+...+50.52\)

\(\Rightarrow6A=2.4.6+4.6.6+...+50.52.6\)

\(=2.4.6+4.6.\left(8-2\right)+....+50.52.\left(54-48\right)\)

\(=2.4.5+4.6.8-2.4.6+...+50.52.54-48.50.52\)

\(=50.52.54\)

\(\Rightarrow A=\frac{50.52.54}{6}=23400\)

\(\text{Đặt A = 2.4 + 4.6 + ... + 50.52}\)

\(\text{⇒6A = 2.4.6 + 4.6.6 + ... + 50.52.6}\)

\(\text{6A= 2.4.6 + 4.6. 8 − 2 + .... + 50.52. 54 − 48 }\)

\(\text{6A= 2.4.5 + 4.6.8 − 2.4.6 + ... + 50.52.54 − 48.50.52}\)

\(\text{6A=50.52.54 }\)

\(\Rightarrow A=\)\(\frac{\text{50.52.54 }}{6}\)\(=23400\)

\(A=\frac{1}{2.4}+\frac{1}{4.6}+\frac{1}{6.8}+...+\frac{1}{48.50}.\)

\(=\frac{1}{2}.\left(\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}....+\frac{2}{48.50}\right)\)

\(=\frac{1}{2}.\left(\frac{4-2}{2.4}+\frac{6-4}{4.6}+\frac{8-6}{6.8}+...+\frac{50-48}{48.50}\right)\)

\(=\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+.....+\frac{1}{48}-\frac{1}{50}\right)\)

\(=\frac{1}{2}.\left(\frac{1}{2}-\frac{1}{50}\right)\)

\(=\frac{1}{2}.\frac{12}{25}=\frac{6}{25}\)

\(B=\frac{3}{1.4}+\frac{3}{4.7}+....+\frac{3}{97.100}\)

\(=\frac{4-1}{1.4}+\frac{7-4}{4.7}+....+\frac{100-97}{97.100}\)

\(=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+.....+\frac{1}{97}-\frac{1}{100}\)

\(=1-\frac{1}{100}=\frac{99}{100}\)

\(C=\frac{8}{7.14}+\frac{8}{14.21}+....+\frac{8}{91.98}\)

\(=\frac{7}{8}.\left(\frac{7}{7.14}+\frac{7}{14.21}+...+\frac{7}{91.98}\right)\)

\(=\frac{7}{8}.\left(\frac{1}{7}-\frac{1}{14}+\frac{1}{14}-\frac{1}{21}+.....+\frac{1}{91}-\frac{1}{98}\right)\)

\(=\frac{7}{8}.\left(\frac{1}{7}-\frac{1}{98}\right)\)

\(=\frac{7}{8}.\frac{13}{98}=\frac{13}{112}\)

gọi tổng của 1+2+3+4+...+79 là M

                     2+3+4+...+80 là N

ta có A = M.N

từ 1 đến 79 hay từ 2 đến 80 có  (79-1) chia 1 + 1=79

M = (79+1).79 chia 2= 3160

N = (80+2).79chia 2= 3239

A = 3160 .3239 = 10235240

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: