\(\dfrac{2}{5}+\dfrac{3}{7}\)=\(\dfrac{14}{35}\)+\(\dfrac{15}{35}\)=\(\dfrac{29}{35}\)

2/5+1/7x3=2/5+3/7

                =14/35+15/35=29/35

\(\dfrac{-6x^4+7x^3+5x+2}{3x+1}\)

\(=\dfrac{-6x^4-2x^3+9x^3+3x^2-3x^2-x+6x+2}{3x+1}\)

\(=\dfrac{-2x^3\left(3x+1\right)+3x^2\left(3x+1\right)-x\left(3x+1\right)+2\left(3x+1\right)}{3x+1}\)

\(=-2x^3+3x^2-x+2\)

\(Sửa:\left(2x^4-7x^3-7x^2-6x-2\right):\left(2x^2+x-1\right)\\ =\left(2x^4+x^3-x^2-8x^3-4x^2+4x-2x^2-x+1-9x-3\right):\left(2x^2+x-1\right)\\ =\left[x^2\left(2x^2+x-1\right)-4x\left(2x^2+x-1\right)-\left(2x^2+x-1\right)-9x-3\right]:\left(2x^2+x-1\right)\\ =x^2-4x-1\left(\text{dư }-9x-3\right)\)

1+2+3+4+5+6=21

2-1x9-7x3-5:1+13= -20

1+2+3+4+5+6 = 21

2-1x9-7x3-5:1+13=(-20)

1 + 2 + 3 + 4 + 5 + 6

= 21

2 - 1 x 9 - 7 x 3 - 5 : 1 + 13

= 2 - 9 - 21 - 5 + 13

= -20

=7x3³-8=7x27-8=181

\(7.3^3-\left(8.1\right)^2=7.27-8^2=189-64=125\)

54/35

\(-\frac{23}{7}.\frac{3}{10}+\frac{13}{7}.\frac{3}{10}\)

\(=\frac{3}{10}.\left(\left(-\frac{23}{7}\right)+\frac{13}{7}\right)\)

\(=\frac{3}{10}.\left(\frac{-10}{7}\right)\)

\(=\frac{-3}{7}\)

=7x(3+3+2+1)

=7x9

=63

7x3+7x2+7x3+7

= 7x (3+2+3+7)

= 7x 15

= 105

7 x 3 + 7 x 2 + 7 x 3 + 7

7 x 3 + 7 x 2 + 7 x 3 + 7 x 1

= 7 x ( 3 + 2 + 3 + 1 ) 

= 7 x 9 

= 63

Chúc bạn học tốt !

a: Ta có: \(\left(x^2+2\right)\left(x-4\right)-\left(x+2\right)^3=-16\)

\(\Leftrightarrow x^3-4x^2+2x-8-x^3-6x^2-12x-8=-16\)

\(\Leftrightarrow-10x^2-10x=0\)

\(\Leftrightarrow-10x\left(x+1\right)=0\)

\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-1\end{matrix}\right.\)

c: Ta có: \(x^3+3x^2+3x+28=0\)

\(\Leftrightarrow\left(x+1\right)^3=-27\)

\(\Leftrightarrow x+1=-3\)

hay x=-4