\(4.S=\frac{4}{5.9}+\frac{4}{9.13}+...+\frac{4}{41.45}\)

\(4S=\frac{1}{5}-\frac{1}{6}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{41}-\frac{1}{45}\)

\(4S=\frac{1}{5}-\frac{1}{45}=\frac{8}{45}\)

\(S=\frac{8}{45}:4=\frac{2}{45}\)

\(S=\frac{1}{5.9}+\frac{1}{9.13}+\frac{1}{13.17}+...+\frac{1}{41.45}\)

\(\Rightarrow4S=\frac{4}{5.9}+\frac{4}{9.13}+\frac{4}{13.17}+...+\frac{4}{41.45}\)

\(\Rightarrow4S=\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{17}+...+\frac{1}{41}-\frac{1}{45}\)

\(\Rightarrow4S=\frac{1}{5}-\frac{1}{45}=\frac{8}{45}\Rightarrow S=\frac{8}{45}:4=\frac{2}{45}\)

~ Hok tốt ~

a)=(3/8+10/16)+(7/12+10/24)

=1+1=2

c)=(4/6+14/6)+(7/13+19/13)+(17/9+1/9)

=3+2+2=7

\(a.=\frac{4}{13}\times\frac{13}{4}\times\frac{2}{3}\times\frac{3}{2}\)

\(=1\times1\)

\(=1\)

\(b.=\frac{19}{13}+\frac{7}{13}+\frac{14}{6}+\frac{4}{6}+\frac{1}{9}+\frac{17}{9}\)

\(=\frac{26}{13}+\frac{18}{6}+\frac{18}{9}\)

\(=2+3+2\)

\(=7\)

4/13 * 2/3 * 13/4* 3/2= (4/13 * 13/4 )* (2/3 * 3*2) = 1*1=1

19/13 + 14/6 + 1/9 + 4/6 + 7/13 + 17/9 = (19/13 + 7/13) + (14/6 + 4/6) + (1/9 + 17/9) = 2 + 3 + 2 =7

ra nhiều thế

a/b nhân 4 cộng 1/6 = 17/6 số phải tìm là bao nhiêu

BAI 1 ; 

Bài 2: 

a, \(\dfrac{5}{23}\) \(\times\) \(\dfrac{17}{26}\) + \(\dfrac{5}{23}\) \(\times\) \(\dfrac{9}{26}\) 

= \(\dfrac{5}{23}\) \(\times\) ( \(\dfrac{17}{26}\) + \(\dfrac{9}{26}\))

= \(\dfrac{5}{23}\) \(\times\) \(\dfrac{26}{26}\)

= \(\dfrac{5}{23}\) 

b, \(\dfrac{3}{4}\) \(\times\) \(\dfrac{7}{9}\) + \(\dfrac{7}{4}\)  \(\times\) \(\dfrac{3}{9}\)

= \(\dfrac{7}{12}\) + \(\dfrac{7}{12}\)

= \(\dfrac{14}{12}\)

= \(\dfrac{7}{6}\)

Tìm x

\(9+\frac{1}{x}+p:89\times97\div9+x=914,978,000\)

=(4/6+14/6)+(7/13+19/13)+(17/9+1/9)

=18/6+26/13=18/9

=3+2+2=7

\(\frac{1}{5\times9}+\frac{1}{9\times13}+\frac{1}{13\times17}+...+\frac{1}{41\times45}\)

= \(\frac{1}{4}\times\left(\frac{4}{5\times9}+\frac{4}{9\times13}+\frac{4}{13\times17}+...+\frac{4}{41\times45}\right)\)

= \(\frac{1}{4}\times\left(\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+\frac{1}{13}-\frac{1}{17}+...+\frac{1}{41}-\frac{1}{45}\right)\)

= \(\frac{1}{4}\times\left(\frac{1}{5}-\frac{1}{45}\right)\)

= \(\frac{1}{4}\times\frac{8}{45}=\frac{2}{45}\)

\(A=\frac{1}{5\cdot9}+\frac{1}{9\cdot13}+...+\frac{1}{41\cdot45}\)

\(4A=\frac{4}{5\cdot9}+\frac{4}{9\cdot13}+...+\frac{4}{41\cdot45}\)

\(4A=\frac{1}{5}-\frac{1}{9}+\frac{1}{9}-\frac{1}{13}+...+\frac{1}{41}-\frac{1}{45}\)

\(4A=\frac{1}{5}-\frac{1}{45}\)

\(4A=\frac{8}{45}\)

\(A=\frac{2}{45}\)

9×20×14×13×11chia hết cho 9 và10 nên y=0. Mà:
3603x0 chia hết cho 9 nên 3+6+0+3+x+0 chia hết cho 9. Suy ra x = 6.

b.1×3×5×7×9×11 chia hết cho 5 và 9.

Mà tích không có số chẵn nên y=5.

Mà: 10x95 chia hết cho 9 nên 1+0+x+9+5 chia hết cho 9 .

Suy ra x=3