Question

[nguyen van giang]

tính tổng 20.21+21.22+22.23+...+39.40

Answer 1

[Dương Lam Hàng]

Ta có: \(S=20.21+21.22+22.23+....+39.40\)

\(\Rightarrow3S=3.20.21+3.21.22+.....+3.39.40\)

\(\Rightarrow3S=20.21.\left(22-19\right)+22.23.\left(24-21\right)+.....+39.40.\left(41-38\right)\)

\(\Rightarrow3S=39.40.41-19.20.21\)

\(\Rightarrow3S=55980\)

\(\Rightarrow S=18660\)

Answer 2

[Trần Minh Khôi]

Cách giải

=(39.40-20.21):0.01+1

=19.19:0.01+1

=1919+1

=1920