Ta có:

\(D=\dfrac{1.2+2.4+3.6+4.8+5.10}{3.4+6.8+9.12+12.16+15.20}\\ =\dfrac{1.2+1.2.4+1.2.3.3+1.2.2.8+1.2.5.5}{3.4+3.4.2.2+3.4.3.3+3.4.2.8+3.4.5.5}\\ =\dfrac{1.2\left(4+3^2+2.8+5^2\right)}{3.4\left(2^2+3^2+2.8+5^2\right)}\\ =\dfrac{1.2}{3.4}=\dfrac{1}{6}\)

Vậy \(D=\dfrac{1}{6}\)

thiếu 

1/6 uyhvnb

ảnh có dễ thươg các bạn!

Ý ban là:

\(\frac{1}{6}\)

= \(\frac{1.2+2.4+3.6+4.8+5.10}{3.2\left(1.2+2.4+3.6+4.8+5.10\right)}=\frac{1}{6}\)
nha :*

cách giải nè.

Ta có: \(\frac{1.2+2.4+3.6+4.8+5.10}{3.4+6.8+9.12+12.16+15.20}\)

=\(\frac{1.2+2.4+3.6+4.8+5.10}{3.2.1.2+3.2.2.4+3.2.3.6+3.2.4.8+3.2.5.10}\)

=\(\frac{1.2+2.4+3.6+4.8+5.10}{3.2.\left(1.2+2.4+3.6+4.8+5.10\right)}\)

=\(\frac{1}{6}\)

\(\frac{1}{6}\)