Question

Tìm X biết:

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

Answer 1

tìm x mà lại có y là sao

 

Answer 2

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 \(Y-\frac{6}{7}=1\Leftrightarrow y=\frac{13}{7}\)