Bài 1 : Tính nhanh a, 34 x 15 + 66 x 71 + 34 x 56 b, 34 x 121 + 61 x 61 -31 x 60

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1. Bài 1: Tính nhanh:

   a)`34×15+66×71+34×56`

   `=(34×15+34×56)+66×71`

   `=34×(15+56)+66×71`

   `=34×71+66×71`

   `=(34+66)×71`

   `=100×71`

   `=7100`

   b)`34×121+61×61-31×60`

   `=34×(60+61)+61×61-31×60`

   `=34×60+34×61+61×61-31×60`

   `=(34×60-31×60)+(34×61+61×61)`

   `=(34-31)×60+(34+61)×61`

   `=3×60+95×61`

   `=3×60+95×60+95`

   `=(3+95)×60+95`

   `=98×60+95`

   `=5880+95`

   `=5975`

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2. $a$) $34 \times 15 + 66 \times 71 + 34 \times 56$

   $= 34 \times ( 15 + 56) + 66 \times 71$

   $= 34 \times 71 + 66 \times 71$

   $= 71 \times (34 + 66)$

   $= 71 \times 100$

   $= 7100$

   $b$) $34 \times 121 + 61 \times 61 – 31 \times 60$

   $= 34 \times 60 + 34 \times 61 + 61 \times 61 – 31 \times 60$

   $= 60 \times (34-31) + 61 \times (34 + 61)$

   $= 60 \times 3 + 61 \times 95$

   $= 180 + 5795$

   $= 5975$

    

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