Bài 1: Tính hợp lí: a) (279 – 1987) + (-18 + 1987 – 279) b) -(3251 + 415) – (-2000 + 585 – 251) c) -25-26-27-28-29+15+16+17+18+19 d) 71 – (-30) – (+18

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1. `(279 – 1987) + (-18 + 1987 – 279)`

   `= 279 – 1987 -18 + 1987 – 279`

   `= (279 – 277) + (1987 – 1987) – 18`

   `= 0 + 0 – 18`

   `= 18`

   `-3251 – 415 +2000 – 585 + 251`

   `= (-3251 + 251) – (415+585) + 2000`

   `= 3000 – 1000 + 2000`

   `= 4000`

   `-25-26-27-28-29+15+16+17+18+19`

   `= (-25+15) + (16-26) + (17-27) + (18-28) + ( 19 – 29)` 

   `= -10 + (-10) + (-10) + (-10) + (-10) `

   `= -50`

   `71 – (-30) – (+18) + (-30) + (+118)`

   `= 71 +30 -18 -30 + 118`

   `= 71 + (30-30) + (118-18) `

   `= 71 + 0 + 100`

   `= 171`

   XIN HAY NHẤT Ạ.

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2. a) (279-1987)+(-18+1987-279)

   =279-1987-18+1987-279

   =(279-279)+(-1987+1987)-18

   =0+0-18

   =-18

   b) -(3251+415)-(-2000+585-251)

   =-3251-415+2000-585+251

   =(-415-585+2000)+(-3215+215)

   =1000-3000

   =-2000

   c)-25-26-27-28-29+15+16+17+18+19

   =(-25+15)+(-26+16)+(-27+17)+(-28+18)+(-29+29)

   =(-10)+(-10)+(-10)+(-10)+(-10)

   =-50

   d) 71-(-30)-(+18)+(-30)+(+118)

   =71+30-18-30+118

   =(30-30)+(-18+118)+71

   =0+100+71

   =171

    

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