Question

1) A= 1/3.4 + 1/4.5 +.....+ 1/39.40

2) B= 1/4.7 + 1/7.10 + .....+ 1/37.40

3) C= 2/4.7 + 2/7.10 +.......+ 2/37.40

Lưu ý: dấu chấm là dấu nhân( nếu bạn nào ko biết)

Mình cần gấp, nhờ các bạn. Phải đúng nhá

Thanks you

Answer 1

1) 

A= \(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{39.40}\)

\(\Rightarrow A=\frac{1}{3}-\frac{1}{4}+\frac{1}{5}-...+\frac{1}{39}-\frac{1}{40}\)

\(\Rightarrow A=\frac{1}{3}-\frac{1}{40}\)

=> A= 27/120

Answer 2

A = \(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{39.40}\)

= \(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{39}-\frac{1}{40}\)

= \(\frac{1}{3}-\frac{1}{40}\)

= \(\frac{37}{120}\)

B = \(\frac{1}{4.7}+\frac{1}{7.10}+...+\frac{1}{37.40}\)

= \(\frac{1}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{37}-\frac{1}{40}\right)\)

= \(\frac{1}{3}\left(\frac{1}{4}-\frac{1}{40}\right)\)

= \(\frac{1}{3}.\frac{9}{40}=\frac{3}{40}\)

C = \(\frac{2}{4.7}+\frac{2}{7.10}+...+\frac{2}{37.40}\)

= \(\frac{2}{3}\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{37}-\frac{1}{40}\right)\)

= \(\frac{2}{3}.\left(\frac{1}{4}-\frac{1}{40}\right)\)

= \(\frac{2}{3}.\frac{9}{40}=\frac{3}{20}\)

Answer 3

1) A = \(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{39.40}\)

A = \(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{39}-\frac{1}{40}\)

A = \(\frac{1}{3}-\frac{1}{40}\)

A = \(\frac{37}{120}\)

2) B = \(\frac{1}{3}.\left(\frac{1}{4}-\frac{1}{7}\right)+\frac{1}{3}.\left(\frac{1}{7}-\frac{1}{10}\right)+...+\frac{1}{3}.\left(\frac{1}{37}-\frac{1}{40}\right)\)

B = \(\frac{1}{3}.\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{37}-\frac{1}{40}\right)\)

B = \(\frac{1}{3}.\frac{9}{40}\)

B = \(\frac{3}{40}\)

3) C = \(\frac{2}{4.7}+\frac{2}{7.10}+...+\frac{2}{37.40}\)

C = \(\frac{2}{3}.\left(\frac{1}{4}-\frac{1}{7}\right)+\frac{2}{3}.\left(\frac{1}{7}-\frac{1}{10}\right)+...+\frac{2}{3}.\left(\frac{1}{37}-\frac{1}{40}\right)\)

C = \(\frac{2}{3}.\left(\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{37}-\frac{1}{40}\right)\)

C = \(\frac{2}{3}.\left(\frac{1}{4}-\frac{1}{40}\right)\)

C = \(\frac{2}{3}.\frac{9}{40}\)

C = \(\frac{3}{20}\)

Answer 4

suy ra A = 1/3-1/4+1/4-1/5+....+1/39-1/40

              = 1/3 - 1/40

              = 37/120

         3B = 3/4.7+3/7.10+....+3/39.40

              = 1/4 -1/7+1/7-1/10+....+1/39-1/40

              = 1/4 - 1/40

              = 9/40

B = 9/40 : 3 = 3/40

     3/2.C =  3/4.7+3/7.10+....+3/39.40

              = 1/4 -1/7+1/7-1/10+....+1/39-1/40

              = 1/4 - 1/40

              = 9/40

C = 9/40 : 3/2 = 3/20

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nhớ tích đó, cảm ơn trước!!!! hihi