ta có 

\(3S=3.20.21+3.21.22+3.22.23+..+3.39.40\)

\(3S=\left(22-19\right).20.21+\left(23-20\right).21.22+..+\left(41-38\right).39.40\)

\(3S=-19.20.21+20.21.22-20.21.22+21.22.23+...+39.40.41\)

\(3S=39.40.41-19.20.21\Rightarrow S=18660\)

Lời giải:

$=20,21\times 10+2021\times 0,4+20,21\times 50$

$=20,21\times 10 +20,21\times 40+20,21\times 50$

$=20,21\times (10+40+50)=20,21\times 100=2021$

202,01

20,21 x 36 + 63 x 20,21 + 20,21

= 20,21 x 36 + 63 x 20,21 + 20,21 x 1

= 20,21 x ( 36 + 63 + 1)

= 20,21 x 100

= 2 021

20,21 x 36 + 63 x 20,21 + 20,21

= 20,21 x (36 + 63 + 1)

= 20,21 x 100

= 2021

\(S=20.21+22.23+23.24+....+39.40\)
\(3S=3.20.21+3.22.23+3.23.24+...+3.39.40\)
\(3S=20.21.\left(22-19\right)+22.23.\left(24-21\right)+....+39.40.\left(41-38\right)\)
\(3S=39.40.41-19.20.21\)
\(3S=55980\)
\(S=18660\).

Lời giải:

$S=10.11+11.12+12.13+...+29.30$

$3S=10.11(12-9)+11.12(13-10)+12.13(14-11)+...+29.30(31-28)$

$=(10.11.12+11.12.13+12.13.14+...+29.30.31)-(9.10.11+10.11.12+...+28.29.30)$

$=29.30.31-9.10.11=25980$

$\Rightarrow S=8660$

Tính bằng cách hợp lý

20,21 + 20,21 x 36 + 20,21 x 63

= 20,21 x ( 1 + 36 + 63 )

= 20,21 x 100

= 2021

20,21 + 20,21 x 36 + 20,21 x 63

=20,21x1 + 20,21 x 36 + 20,21 x 63

=20,21x(1+36+63)

=20,21x100

=2021

20,21 + 20,21 x 36 + 20,21 x 63

= 20,21 x 1 + 20,21 x 36 + 20,21 x 63

= 20,21 x ( 1 + 36 + 63 )

= 20,21 x 100

= 2021