3/3x6 + 3/6x9 + 3/9x12 + 3/12x15 + 3/15x18

= 1/3 - 1/6 + 1/6 - 1/9 + ... + 1/15 - 1/18

= 1/3 - 1/18

= 5/18

\(\frac{3}{3\times6}+\frac{3}{6\times9}+\frac{3}{9\times12}+\frac{3}{12\times15}+\frac{3}{15\times18}\)

=\(3\times\left(\frac{1}{3\times6}+\frac{1}{6\times9}+\frac{1}{9\times12}+\frac{1}{12\times15}+\frac{1}{15\times18}\right)\)

=\(3\times\left(\frac{1}{3}\times\frac{1}{6}+\frac{1}{6}\times\frac{1}{9}+\frac{1}{9}\times\frac{1}{12}+\frac{1}{12}\times\frac{1}{15}+\frac{1}{15}\times\frac{1}{18}\right)\)

=\(3\times\left(\frac{1}{3}\times\frac{1}{18}\right)\)

=\(3\times\frac{1}{54}=\frac{3}{54}=\frac{1}{18}\)

\(3x^6-4x^5+2x^4-8x^3+2x^2-4x+3\)

\(=3x^6+3x^4-4x^5-4x^3-x^4-x^2-4x^3-4x+3x^2+3\)

\(=\left(x^2+1\right)\left(3x^4-4x^3-x^2-4x+3\right)\)

\(=\left(x^2+1\right)\left(x^2+x+1\right)\left(3x^2-7x+3\right)\)

Chọn C

6 x 9 - 2 x 6 + 8 x 3 : 2 x 3

= 6 x 9 : 2 x 3 + - 2 x 6 : 2 x 3 + 8 x 3 : 2 x 3

=  3 x 6 - x 3 + 4

khó thế ai làm được? ?????????

:v thế mới đi hỏi 

\(\frac{2}{1.3}+\frac{3}{3.6}+\frac{4}{6.10}+\frac{5}{10.15}+\frac{6}{15.21}\)

= \(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{6}+\frac{1}{6}-\frac{1}{10}+\frac{1}{10}-\frac{1}{15}+\frac{1}{15}-\frac{1}{21}\)

= \(1-\frac{1}{21}\)

= \(\frac{20}{21}\)

Ủng hộ mk nha !!! ^_^

3 x 6 = 18

k mk mk k lại

3x6=18