2.8 x 1.2 + 4.8 x 1.8

= 2.8 x 1.2 + 1,2 x 4  x 1.8

= 2.8 x 1.2 + 1,2 x 7,2

= 1,2 x ( 2,8 + 7,2 )

= 1,2 x 10

= 12

2.8 x 1.2 + 4.8 x 1.8

= 2.8 x 1.2+1.2 x 4 x 1.8

= 2.8 x 1.2 + 1.2 x 7.2

= 1.2 (2.8+7.2)

= 1.2 x 10

= 12 

2.8 x 1.2 + 4.8 x 1.8

= 3.36 + 8.64

= 29.0304

Ta có:

\(\frac{1.2+2.4+3.6+4.8+5.10+6.12}{3.4+6.8+9.12+12.16+15.20}\)

\(=\frac{1.2+2\left(1.2\right)+3.\left(1.2\right)+4\left(1.2\right)+5.\left(1.10\right)+6\left(1.2\right)}{3.4+2\left(3.4\right)+3\left(3.4\right)+4\left(3.4\right)+5\left(3.4\right)+6\left(3.4\right)}\)

\(=\frac{\left(1.2\right)\left(1+2+3+4+5+6\right)}{\left(3.4\right)\left(1+2+3+4+5+6\right)}\)

\(=\frac{1.2}{3.4}\)

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

Ai k mình mình k lại.

= 2,8 x 1,2 + ( 2 + 2,8 ) x 1,8

= 2,8 x 1,2 + 2 x 1,8 + 2,8 x 1,8

= 2,8 x ( 1,8 + 1,2 ) + 2 x 1,8

= 2 ,8 x 3 + 3,6

= 8,4 + 3 ,6

= 12

=2x(8+1)+8x(4+1)

=2x9+8x5

=18+40

=58

Ta có : 

2,8 x 1,2 + 4,8 x 1,8 

= 2,8 x 1,2 + ( 2,8 + 2 ) x 1,8 

= 2,8 x 1,2 + 2,8 x 1,8 + 2,8 x 1 

= 2,8 x ( 1,2 + 1,8 + 1 ) 

= 2,8 x 4 

= 11,2 

Đề hỏi cái gì ??

So sánh nha

\(1\cdot2+2\cdot3+3\cdot4+...+n\left(n+1\right)\\ =\dfrac{1}{3}\left[1\cdot2\cdot3+2\cdot3\cdot3+...+3n\left(n+1\right)\right]\\ =\dfrac{1}{3}\left[1\cdot2\left(3-0\right)+2\cdot3\left(4-1\right)+...+n\left(n+1\right)\left(n+2-n+1\right)\right]\\ =\dfrac{1}{3}\left[1\cdot2\cdot3-1\cdot2\cdot3+2\cdot3\cdot4-...-\left(n-1\right)n\left(n+1\right)+n\left(n+1\right)\left(n+2\right)\right]\\ =\dfrac{n\left(n+1\right)\left(n+2\right)}{3}\)

. là dấu nhân hay dấu phẩy vậy bạn

b)[1,1*(-4,3+4,5)]:(-10+10,01)

(1,1*0,2):0,01

=0,22:0,01

=22

c)=(6,7+5,66-3,7+4,34)*(-76,6*1,2+7,66*10*1,2)

=(6,7+5,66-3,7+4,34)*(-76,6*1,2+76,6*1.2)

=(6,7+5,66-3,7+4,34)*[1,2*(-76,6+76,6)]

=(6,7+5,66-3,7+4,34)*(1,2*0)

=(6,7+5,66-3,7+4,34)*0

=0

\(a.\)

\(6,5+1,2+3,5-5,2+6,5-4,8\)

\(=\left(-5,2+1,2\right)+\left(3,5+6,5\right)+\left(6,5-4,8\right)\)

\(=\left(-4\right)+10+1,7\)

\(=6+1,7\)

\(=7,7\)

\(b.\)

\(\left(-4,3.1,1+1,1.4,5\right):\left(-0,5:0,05+10,01\right)\)

\(=\left[1,1.\left(-4,3+4,5\right)\right]:\left(-10+10.01\right)\)

\(=\left[1,1.0,2\right]:\left(0,01\right)\)

\(=0,22:0,01\)

\(=22\)

\(c.\)

\(\left(6,7+5,66-3,7+4,34\right).\left(-76,6.1,2+7,66.12\right)\)

\(=\left(6,7+5,66-3,7+4,34\right).\left[76,6\left(-1,2\right)+7,66.12\right]\)

\(=13.0\)

\(=0\)

= -3,5 nha bạn

(Xin lỗi mình không giải thẳng được)

~Chúc bạn học tốt~