Lời giải:

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

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

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

 = (0,1+3,9) + (0,3+3,7)+....+(1,9+2,1) ( có 10 cặp )

 = 4+4+...+4 ( 10 số 4) = 4x 10 = 40

k mk nha

= (0,1+3,9)+(0,3+3,7)+(0,5+3,5)+(0,7+3,3)+(0,9+3,1)+(1,1+2,9)+(1,3+2,7)+(1,5+2,5)+(1,7+2,3)+(1,9+2,1)+2

= 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 +2

= 4 x 10 + 2

=40 + 2

=42  

bạn làm bài nay rùi à nếu làm rùi mk hỏi thêm câu 

87,4 x 12,74 + 74,52 x 43,7

56,27 x 115 - 56.27 x 0.1 - 56.27 : 0.2 

= ( 115 - 0.1 ) x 56.27 - 56.27 : 0.2

=     6465.423 - 56.27 : 0.2

= 6465.423 - 281.35

= 6184.073

=56,27 x (115-0,1:0.2)

=56,27 x 114,5

=6442,915

a) ( -17 + |91| ) – ( 91 – |-17| – 2011 )

= ( -17 + 91 ) – ( 91 – 17 – 2011 )

= - 17 + 91 – 91 + 17 + 2011

= ( - 17 + 17 ) + (91 – 91 ) + 2011 = 2011

a)x . 3,5 + x . 12,5 = 396,8
   x (3,5+12,5) =396,8 
   x. 16 =396,8
  x = 396,8 :16
 x= 24,8
b) 20,11 : 0,1 + 20,11: 10 -20,11 : (2,1 : 21) =2,011

\(a,-215-358+215+\left(-42\right)\\ =\left(-215+215\right)+\left(-358-42\right)\\ =0-400\\ =-400\\ b,13\cdot\left(23+22\right)-3\cdot\left(17+28\right)\\ =13\cdot45-3\cdot45\\ =45\cdot\left(13-3\right)\\ =45\cdot10\\ =450\)

a) ( -17 + |91| ) – ( 91 – |-17| – 2011 ) = ( -17 + 91 ) – ( 91 – 17 – 2011 )

= - 17 + 91 – 91 + 17 + 2011 = ( - 17 + 17 ) + (91 – 91 ) + 2011 = 2011

A.\(=\frac{17}{25}\times\left(\frac{13}{19}+\frac{6}{19}\right)\)

\(=\frac{17}{25}\times1\)

\(=\frac{17}{25}\)

B\(=\frac{47}{19}:\left(\frac{15}{32}-\frac{15}{17}\right)\)

\(=\frac{47}{19}:1\)

\(=\frac{47}{19}\)

\(27,3.4,6+96,4.27,3-27,3\)

\(=27,3.\left(4,6+96,4-27,3\right)\)

\(=27,3.73,56\)

\(=2008,188\)