\(\frac{20}{1}< \frac{4}{5}\)

a = 1 .

k cho mình nha tiến đạt mình k cho bạn rùi

\(S=1.2+2.3+...+39.40\)

\(3S=1.2.\left(3-0\right)+2.3\left(4-1\right)+...+39.40\left(41-38\right)\)

\(3S=1.2.3-0.1.2+2.3.4-1.2.3+...+39.40.41-38.39.40\)

\(S=\frac{39.40.41}{3}\)

\(\Rightarrow S=21320\)

chuong do co 13 con ga 

2 + 2 + 2 + 2+2 = 10

150+300

=450