a) 7/2.x - (7.(18+45+47))=3

   7/2.x - 7.110 = 3

   7/2.x - 770 =3 => 7/2.x = 3+770=773

                         => x = 773 : 7/2 = 1546/7

b) 19/3 - ( 4x+6x+x)= 4/7 : 2/5

19/3 - 11x = 10/7

11x = 19/3 - 10/7 = 103/21

x = 103/21 : 11 = 103/231

3/8 x 5/4 =15/32

4/7 x 2/13=8/91

cách làm là lấy tử nhân tử mẫu nhân mẫu 

cái này học từ lớp 4 rồi còn gì

3/8*5/4=15/32

4/7*2/13=8/91

\(\frac{3}{8}\)* \(\frac{5}{4}\)= \(\frac{15}{32}\)                                   \(\frac{4}{7}\)* \(\frac{2}{13}\)= \(\frac{8}{91}\)

Bạn chỉ cần lấy tử số nhân tử số , mẫu số nhân mẫu số thôi

x*3/5+x*2/7=31/70

x*(3/5+2/7) =31/70

x*31/35      =31/70

         x       =31/70 :31/35

         x        = 1/2

\(x.\frac{3}{5}+x.\frac{2}{7}=\frac{31}{70}\)

\(x\left(\frac{3}{5}+\frac{2}{7}\right)=\frac{31}{70}\)

\(x\left(\frac{21}{35}+\frac{10}{35}\right)=\frac{31}{70}\)

\(\frac{31}{35}x=\frac{31}{70}\)

\(x=\frac{31}{70}:\frac{31}{35}\)

\(x=\frac{31}{70}.\frac{35}{31}\)

\(x=\frac{35}{70}\)

\(x=\frac{1}{2}\)

vay \(x=\frac{1}{2}\)

1+1+2+3+3+4+4+5+5+6+6+7+7+8+8+9+9

= 2 x ( 1+2+3+4+5+6+7+8+9)

= 2 x 45

= 90

=2*(1+2+3+4+5+6+7+8+9)

=2*45

=90

           k mình nha !!!

1 + 1 + 2 + 2 + 3 + 3 + 4 + 4 + 5 + 5 + 6 + 6 + 7 + 7 + 8 + 8 + 9 + 9 = 

= 2 x (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9)

= 2 x 45

= 90

4x5x0,25x1/5x1/2x2=(4x0,25)x(2x5)x(1/5x1/2)=1x10x1/10=1x1=1

( 4 x 0,25 ) x ( 5 x 1/5 ) x ( 1/2 x 2 )

= 1 x 1 x 1 = 1

\(a,=\frac{7-1}{1.3.7}+\frac{9-3}{3.7.9}+\frac{13-7}{7.9.13}+\frac{15-9}{9.13.15}\)\(+\frac{19-13}{13.15.19}\)

\(=\frac{1}{1.3}-\frac{1}{3.7}+\frac{1}{3.7}-\frac{1}{7.9}+\frac{1}{7.9}-\frac{1}{9.13}+\frac{1}{9.13}-\frac{1}{13.15}+\frac{1}{13.15}-\frac{1}{15.19}\)

\(=\frac{1}{1.3}-\frac{1}{15.19}=\frac{95}{285}-\frac{1}{285}=\frac{94}{285}\)

\(b,=\frac{1}{6}.\left(\frac{6}{1.3.7}+\frac{6}{3.7.9}+\frac{6}{7.9.13}+\frac{6}{9.13.15}+\frac{6}{13.15.19}\right)\)

làm giống như trên

\(c,=\frac{1}{8}.\left(\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+\frac{2}{3.4.5}+...+\frac{2}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{3-1}{1.2.3}+\frac{4-2}{2.3.4}+\frac{5-3}{3.4.5}+...+\frac{50-48}{48.49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+\frac{1}{3.4}-\frac{1}{4.5}+...+\frac{1}{48.49}-\frac{1}{49.50}\right)\)

\(=\frac{1}{16}.\left(\frac{1}{2}-\frac{1}{2450}\right)=\frac{1}{16}.\left(\frac{1225}{2450}-\frac{1}{2450}\right)=\frac{153}{4900}\)

\(d,=\frac{5}{7}.\left(\frac{7}{1.5.8}+\frac{7}{5.8.12}+\frac{7}{8.12.15}+...+\frac{7}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{8-1}{1.5.8}+\frac{12-5}{5.8.12}+\frac{15-8}{8.12.15}+...+\frac{40-33}{33.36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{1.5}-\frac{1}{5.8}+\frac{1}{5.8}-\frac{1}{8.12}+\frac{1}{8.12}-\frac{1}{12.15}+...+\frac{1}{33.36}-\frac{1}{36.40}\right)\)

\(=\frac{5}{7}.\left(\frac{1}{5}-\frac{1}{1440}\right)=\frac{5}{7}.\left(\frac{288}{1440}-\frac{1}{1440}\right)=\frac{41}{288}\)

P/S: . là nhân nha

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Cái này dễ thế mà ko bít lm