Question

[Lê Nguyễn Ngọc Nhi]

Tính :

\(B=2\dfrac{1}{315}.\dfrac{1}{651}-\dfrac{1}{105}.3\dfrac{650}{651}-\dfrac{4}{315.651}+\dfrac{4}{105}\)

Mọi người xem thử mk có sai đề ko z?

Answer 1

[Khôi Bùi]

Đề đúng nhé bạn :

\(B=2\dfrac{1}{315}.\dfrac{1}{651}-\dfrac{1}{105}.3\dfrac{650}{651}-\dfrac{4}{315.651}+\dfrac{4}{105}\)

\(\Leftrightarrow B=\left(2+\dfrac{1}{315}\right).\dfrac{1}{651}-\dfrac{3}{315}\left(3+\dfrac{650}{651}\right)-\dfrac{4}{315.651}+\dfrac{12}{315}\)

\(\Leftrightarrow B=\left(2+\dfrac{1}{315}\right).\dfrac{1}{651}-3.\dfrac{1}{315}\left(4-\dfrac{1}{651}\right)-4.\dfrac{1}{315}.\dfrac{1}{651}+12.\dfrac{1}{315}\)

Đặt \(\dfrac{1}{315}=a;\dfrac{1}{651}=b\) thay vào B , ta được :

\(B=\left(2+a\right)b-3a\left(4-b\right)-4ab+12a\)

\(\Leftrightarrow B=2b+ab-12a+3ab-4ab+12a\)

\(\Leftrightarrow B=2b+\left(ab+3ab-4ab\right)+\left(12a-12a\right)\)

\(\Leftrightarrow B=2b\)

\(\Leftrightarrow B=2.\dfrac{1}{651}\)

\(\Leftrightarrow B=\dfrac{2}{651}\)

Vậy \(B=\dfrac{2}{651}\)

:D