This novel………………in the seventeenth century.

A. is written

B. is writing

C. was written

D. was writing

số lớn nhất 999

Số bé nhất 100

có: 999 - 100 + 1 = 900 số 

Đặt \(\dfrac{a}{b}=\dfrac{c}{d}=k\) \(\left(k\ne0\right)\)

\(\Rightarrow a=bk\) ; \(c=dk\)

Thay \(a=bk\) vào \(\dfrac{a}{3a+b}\) ta có:

\(\dfrac{bk}{3bk+b}=\dfrac{bk}{b\left(3k+1\right)}=\dfrac{k}{3k+1}\) (1)

Thay \(c=dk\) vào \(\dfrac{c}{3c+d}\) ta có:

\(\dfrac{dk}{3dk+d}=\dfrac{dk}{d\left(3k+1\right)}=\dfrac{k}{3k+1}\) (2)

Từ (1) và (2) \(\Rightarrow\dfrac{a}{3a+b}=\dfrac{c}{3c+d}\)