a ) 35 24 < 2. b ) 17 56 > − 5 21

a ) 35 24 < 2. b ) 17 56 > − 5 21 .

a: 1825-578+2578-825

=(1825-825)+(2578-578)

=1000+2000

=3000

b: 171-39+29-61

=(171-61)-(39-29)

=110-10

=100

\(275\times15+825+82\times275\)

\(=275\times15+275\times3+82\times275\)

\(=275\times\left(15+3+82\right)\)

\(=275\times100\)

\(=27500\)

=275x15+82x275+825

=275x15+82x275+3x275

=275x(15+82+3)

=275x100

=275

a,     54 x 275 + 825 x 15 + 275 = 54 x 275 + 3 x 275 x 15 + 275

                                              = 275( 54 + 3 x 15 + 1 )

                                             = 275 x 100

                                              = 27500

b , 19 x 29 x 39 x 49 x 59 x 69 x 79 x 89 x 99    có tận cùng là 9

345\691>413\825

Giải 

Ta có: \(\frac{345}{691}\)<    \(\frac{345}{690}\)

            \(\frac{345}{690}\)= \(\frac{1}{2}\)=     \(\frac{413}{826}\)       \(\frac{1}{2}\)

              \(\frac{413}{826}\)< \(\frac{413}{825}\)

 Vậy      \(\frac{345}{691}\)<\(\frac{413}{825}\)

`a) 127 + 29 + 73`

`= (127+73)+29`

`= 200 + 29`

`= 229`

`c) 135 + 360 + 65 + 40`

`= (135+65)+(360+40)`

`= 200+400`

`= 600`

`c) 417-17-299`

`= (417-17)-299`

`= 400 - 299`

`= 101`

`d) 981-781+29`

`= (981-781)+29`

`= 200 + 29`

`= 229`

a) 127 + 29 + 73 = 229

b) 135 + 360 + 65 + 40 = 600

c) 417 - 17 - 299 = 104

d) 981 - 781 + 29 = 229

Chúc bạn học tốt!! ^^

Đặt \(A=5+\frac53+\frac59+\cdots+\frac{5}{729}+\frac{5}{2187}\)

=>\(3\cdot A=15+5+\frac53+\cdots+\frac{5}{243}+\frac{5}{729}\)

=>3A-A=\(15+5+\frac53+\cdots+\frac{5}{243}+\frac{5}{729}-5-\frac53-\frac59-\cdots-\frac{5}{2187}\)

=>2A=\(15-\frac{5}{2187}=\frac{15\cdot2187-5}{2187}=\frac{32800}{2187}\)

=>\(A=\frac{16400}{2187}\)