9x6=54

9x5=45

9x4=36

9x3=27

9x2=18

9x1=9

k mình nhé chúc năm mới vui vẻ ^.^

9x6=54

9x5=45

9x4=36

9x3=27

9x2=18

Ta có: \(1+3+6+10+\cdots+45+55\)

\(=\frac12\times\left(2+6+12+20+\cdots+90+110\right)\)

\(=\frac12\times\left(1\times2+2\times3+\cdots+9\times10+10\times11\right)\)

\(=\frac12\times\left\lbrack1\times\left(1+1\right)+2\times\left(2+1\right)+\cdots+10\times\left(10+1\right)\right\rbrack\)

\(=\frac12\times\left\lbrack\left(1\times1+2\times2+\cdots+10\times10\right)+\left(1+2+\cdots+10\right)\right\rbrack\)

\(=\frac12\times\left\lbrack10\times\left(10+1\right)\times\frac{\left(2\times10+1\right)}{6}+10\times\frac{11}{2}\right\rbrack\)

\(=\frac12\times\left\lbrack10\times11\times\frac{21}{6}+5\times11\right\rbrack=\frac12\times\left\lbrack5\times11\times7+5\times11\right\rbrack\)

\(=\frac12\times5\times11\times\left(7+1\right)=\frac{55}{2}\times8=55\times4=220\)

Ta có: \(1\times10+9\times2+\cdots+10\times1\)

\(=2\times\left(1\times10+2\times9+3\times8+4\times7+5\times6\right)\)

\(=2\times\left\lbrack1\times\left(11-1\right)+2\times\left(11-2\right)+3\times\left(11-3\right)+4\times\left(11-4\right)+5\times\left(11-5\right)\right\rbrack\)

\(=2\times\left\lbrack11\times\left(1+2+3+4+5\right)-\left(1\times1+2\times2+3\times3+4\times4+5\times5\right)\right\rbrack\)

\(=2\times\left\lbrack11\times15-\left(1+4+9+16+25\right)\right\rbrack\)

=2x(165-55)

=2x110

=220

Ta có: \(\frac{1\times10+9\times2+\cdots+10\times1}{1+3+6+10+\cdots+45+55}\)

\(=\frac{220}{220}\)

=1

a =  57/10

b = 12/5

c = 10/3

a) 57/10

b)12/5

c)10/3

2 x 2 + 2 x 2 x 9 + 9 x 5

= 4 + 36 + 45

= 40 + 45

= 85

    2 x 2 + 2 x 2 x 9 + 9 x 5

= 4 + 4 x 9 + 9 x 5

= 4 x ( 1 + 9 )  + 9 x 5

= 4 x10 + 9 x 5

= 4 x 2 x 5 + 9 x 5

= 6 x 5 + 9 x 5

= 5 x ( 6 + 9 )

= 5 x 15

= 75 

1+2+3+4+5+6+7+8+9x5

=36+45

=81

x+ 8/9 = 96/36 : 5

x + 8/9 = 8/15

x          = 8/15- 8/9

x          = -16/45

x + 8/9 x 5 = 96/36

x + 8/9 = 96/36 : 9

x + 8/9 = 8/27

x = -16/27

75/45 = 5/3 nha

a) \(1678\cdot24+77\cdot1678-1678=1678\left(24+77-1\right)=167800\)

b) Ta có: \(\dfrac{2}{5}+\dfrac{4}{11}+\dfrac{3}{5}+\dfrac{18}{11}+\dfrac{1}{4}+\dfrac{1}{6}\)

\(=\left(\dfrac{2}{5}+\dfrac{3}{5}\right)+\left(\dfrac{4}{11}+\dfrac{18}{11}\right)+\dfrac{1}{4}+\dfrac{1}{6}\)

\(=3+\dfrac{1}{4}+\dfrac{1}{6}\)

\(=\dfrac{36}{12}+\dfrac{3}{12}+\dfrac{2}{12}=\dfrac{41}{12}\)