tinh bang cach thuan tien : a, 12*3+12*4+12*5-12*2 b, 2215+2246+2285+2262+2238+2254 c, 5+7+9+11+13+15+17+19+21+23+25

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1. @Bơ

   `12.3+12.4+12.5-12.2`

   `=12.(3+4+5-2)`

   `=12.10`

   `=120`

   `2215+2246+2285+2262+2238+2254`

   `=(2215+2285)+(2246+2254)+(2262+2238)`

   `=4500+4500+4500`
   

   `=13500`

   `5+7+9+11+13+15+17+19+21+23+25`

   `=(5+15)+(7+13)+(9+11)+(17+23)+(19+21)+25`

   `=20+20+20+40+40+25`

   `=(40+40+20)+40+25`

   `=100+40+25`

   `=165`

    

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2. Giải thích các bước giải:

   `a, 12×3+12×4+12×5-12×2`

   `=12×(3+4+5-2)`

   `=12×(10)`

   `=120`
   `b, 2215+2246+2285+2262+2238+2254`

   `=2000×6+(15+46+85+62+38+54)`

   `=12000+300`

   `=12300`
   `c, 5+7+9+11+13+15+17+19+21+23+25`

   `=5+5+2+5+4+5+6+5+8+5+10+5+12+5+14+5+16+5+18+5+20`

   `=11×5+110`

   `=55+110`

   `=165`

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