Question

[ĐoànThùyDuyên]

Tính giá trị của biểu thức sau bằng cách thay số bởi chữ 1 cách hợp lý:

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

Answer 1

[Trần Minh An]

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

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

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

Đặt \(\dfrac{1}{315}=a,\dfrac{1}{651}=b\)

\(\Rightarrow S=\left(2+a\right).b-3a.\left(4-b\right)-4ab+12a\)

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

\(=2b=\dfrac{2}{651}\)