90.2+45.6+15.18=a

180 + 270 + 270 = a

720 = a

=> a = 720

A = 6/3 . ( 1/15.18 + 1/18.21 + 1/21/24 + . . . + 1/87.90 )

A = 6/3 . ( 1/15 - 1/18 + 1/18 - 1/21 + 1/21 - 1/24 + . . . + 1/87 - 1/90 )

A = 2 . ( 1/15 - 1/90 ) 

A = 2. 5/90

A = 10/90 = 1/9

\(\frac{6}{15.18}+\frac{6}{18.21}+\frac{6}{21.24}+...+\frac{6}{84.87}+\frac{6}{87.90}\)

\(=\frac{6}{3}\left(\frac{3}{15.18}+\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{84.87}+\frac{3}{87.90}\right)\)

\(=2\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+...+\frac{1}{84}-\frac{1}{87}+\frac{1}{87}-\frac{1}{90}\right)\)

\(=2\left(\frac{1}{15}-\frac{1}{90}\right)=2\left(\frac{6-1}{90}\right)=2\times\frac{1}{18}=\frac{1}{9}\)

=  225.55 +  225.55

=  451.1 

a)\(65.3,47+3,47.65\)
\(=65.\left(3,47+3,47\right)\)
\(=65.6,94\)
\(=451,1\)

\(65\times\left(3,47+3,47\right)=65\times6,94=451,1\)

=> A = 6/3.( 1/15 - 1/18 + 1/18 - 1/21 + ..... + 1/87 - 1/90 )

=> A = 2.( 1/15 - 1/90 )

=> A = 2.5/90

=> A = 10/90 = 1/9

gfdddddddddddddrh

a: \(78\cdot82=80^2-4=6400-4=6396\)

b: \(45^2+90\cdot55+55^2=100^2=10000\)

`30 xx 25 xx 7 xx8`

`=(30 xx7) xx (25xx8)`

`=210 xx200`

`=42000`

`----`

`75 xx 8 xx 12 xx 14`

`=(75xx8) xx(12xx14)`

`=600xx168`

`=100800`

\(a.30\times25\times7\times8=\left(30\times7\right)\times\left(25\times8\right)=210\times200=42000\)

\(b.75\times8\times12\times14=\left(75\times12\right)\times\left(8\times14\right)=900\times112=100800\)

\(a,30.25.7.8\\ =\left(30.7\right).\left(25.8\right)\\ =210.200\\ =42000\\ b,75.8.12.14\\ =\left(75.12\right).\left(8.14\right)\\ =900.112\\ =100800\)

Ta có : \(\frac{1}{15.18}+\frac{1}{18.21}+\frac{1}{21.24}+...+\frac{1}{87.90}\)

= \(\frac{1}{3}\left(\frac{3}{15.18}+\frac{3}{18.21}+\frac{3}{21.24}+...+\frac{3}{87.90}\right)\)

= \(\frac{1}{3}\left(\frac{1}{15}-\frac{1}{18}+\frac{1}{18}-\frac{1}{21}+\frac{1}{21}-\frac{1}{24}+...+\frac{1}{87}-\frac{1}{90}\right)\)

= \(\frac{1}{3}\left(\frac{1}{15}-\frac{1}{90}\right)\)

= \(\frac{1}{3}.\frac{1}{18}\)

= \(\frac{1}{54}\)

\(\frac{1}{15.18}+\frac{1}{18.21}+....+\frac{1}{87.90}\)

\(=\frac{1}{3}.\left(\frac{1}{15}-\frac{1}{90}\right)=\frac{1}{54}\)

= 1-1/15+(1/18-1/18) + (1/21-1/21)+...+(1/87-1/87) +1/90

= 1-(1/15-1/90)= 1-1/18 = 17/18

duyệt đi

=8,95*90+89,5

=89,5(9+1)

=895