Y=\(\frac{13}{7}\)

\(y-\frac{1}{2}-\frac{1}{6}-\frac{1}{12}-\frac{1}{20}-\frac{1}{30}-\frac{1}{42}=1\)

\(y-\frac{210}{420}-\frac{70}{420}-\frac{35}{420}-\frac{21}{420}-\frac{14}{430}-\frac{10}{420}=1\)

\(y-\frac{210-70-35-21-14-10}{420}=1\)

\(y-\frac{60}{420}=1\)

\(y-\frac{1}{7}=1\)

\(y=1+\frac{1}{7}\)

\(y=\frac{7}{7}+\frac{1}{7}\)

\(y=\frac{8}{7}\)

tìm x mà lại có y là sao

Y - 1/2 - 1/6 - 1/12 - 1/20 - 1/30 - 1/42 = 1

Đặt A= - 1/2 - 1/6 - 1/12 - 1/20 - 1/30 - 1/42 

\(A=-\left(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{42}\right)\)

\(A=-\left(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{6.7}\right)\)

\(A=-\left(1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{6}-\frac{1}{7}\right)\)

\(A=-\left(1-\frac{1}{7}\right)\)

\(A=-\frac{6}{7}\).Thay A vào ta có \(Y-\frac{6}{7}=1\Leftrightarrow y=\frac{13}{7}\)

\(y-\dfrac{1}{2}-\dfrac{1}{6}-\dfrac{1}{12}-\dfrac{1}{20}-\dfrac{1}{30}-\dfrac{1}{42}=1\)

\(y-\dfrac{6}{7}=1\)

\(y=\dfrac{13}{7}\)

1/2 + 1/6 + 1/12 + 1/20 + 1/30 1/x = 41/42

5/6 + 1/x = 41/42

1/x = 41/42 - 5/6 

1/x = 1/7 

vậy x = 7

x = 7 ở vong 18 đúng không

chuẩn 

x + 1/2 + 1/6 + 1/12 + 1/20 + 1/30 + 1/42 = 1 

x + 1/1*2 + 1/2*3 + 1/3*4 + 1/4*5 + 1/5*6 + 1/6*7 = 1 

x + 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 = 1 

x + 1/1 - 1/7 = 1 

x + 6/7 = 1 

x = 1 - 6/7 

x = 1/7 

x + 1/2 + 1/6 + 1/20 + 1/30 + 1/42 = 1

x + 65/84 = 1

x = 1 - 65/84

x = 19/84

x + 1/2 + 1/6 + 1/12 + 1/20 + 1/30 × 1/42 = 1

<=> x + 6/7 = 1

<=> x = 1 - 6/7

<=> x = 1/7

 Vậy x = 1/7

em lớp 6  nha

B= 1/2 + 1/6 + 1/12 +1/20 + 1/30 + 1/42 + 1/56 + 1/72

B= 1/1*2 + 1/2*3 + 1/3*4 + 1/4*5 + 1/5*6 + 1/6*7 + 1/7*8 + 1/8*9

B=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9

B=1+0-0-0-0-0-0-0-1/9

B=1-1/9

B=8/9

k em nha

phần A em chịu

còn câu dưới đó e

bằng 7 dúng không

\(\frac{1}{x}=\frac{41}{42}-\frac{1}{30}-\frac{1}{20}-\frac{1}{12}-\frac{1}{6}-\frac{1}{2}\)

\(\frac{1}{x}=\frac{1}{7}=>x=7\)