Đáp án:

`a) 5/11 + 3/4 = 20/44 + 33/44 = 53/44`

`b) 4/9 xx 3/7 = ( 4 xx 3 ) / ( 9 xx 7 ) = ( 4 xx 1 ) / ( 3 xx 7 ) = 4/21`

`c) 2/5 : 4/7 + 1/3 = 2/5 . 7/4 + 1/3 = 7/10 + 1/3 = 21/30 + 10/30 = 31/30`

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`a) 5/7 < 3/4`

Quy đồng `:`

`5/7 = ( 5 xx 4 ) / ( 7 xx 4 ) = 20/28`

`3/4 = ( 3 xx 7 ) / ( 4 xx 7 ) = 21/28`

Mà  `20/28 < 21/28` `->` `5/7 < 3/4`

`b) 5/13 = 25/65`

Rút gọn `:` `25/65 = 5/13` 

Mà  `5/13 = 5/13` `->` `5/13 = 25/65`

`c) 11/12 < 12/13`

Quy đồng `:`

`11/12 = ( 11 xx 13 ) / ( 12 xx 13 ) = 143/156`

`12/13 = ( 12 xx 12 ) / ( 13 xx 12 ) = 144/156`

Mà  `143/156 < 144/156` `->` `11/12 < 12/13`

`d) 5/6 > 4/5`

Quy đồng `:`

`5/6 = ( 5 xx 5 ) / ( 6 xx 5 ) = 25/30`

`4/5 = ( 4 xx 6 ) / ( 5 xx 6 ) = 24/30`

Mà  `25/30 > 24/30` `->` `5/6 > 4/5`

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Hiệu của  `516273`  và  `394857` `:` `516273 - 394857 = 121 416`

`->` Không có đáp án đúng

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Ta có `:`

`25m^2 = 25 00 00 cm^2`

`7dm^2 = 700cm^2`

`->` `25m^2` `7dm^2` `=` `25 0 700 cm^2`

`->` Chọn  `C`

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Quy đồng `:`

`5/6 = ( 5 xx 5 ) / ( 6 xx 5 ) = 25/30`

`4/5 = ( 4 xx 6 ) / ( 5 xx 6 ) = 24/30`

Mà  `25/30 > 24/30`

`->` `5/6 > 4/5`

`->` Chọn  `D`