Question

Tính nhanh : A = 5.6 + 6.7 + ... + 30.31

Answer 1

           A.3=5.6.3+6.7.3+...+30.31.3

           A.3=5.6.(7-4)+6.7.(8-5)+...+30.31.(32-29)

          A.3=5.6.7-4.5.6+6.7.8-5.6.7+...+30.31.32-29.30.31

          A.3=(5.6.7-5.6.7)+...+(29.30.31-29.30.31)+(30.31.32-4.5.6)

         A.3=0+...+0+30.31.32-4.5.6

         A.3=30.31.32-4.5.6

        A=30.31.32-4.5.6 /3

        A=(29760-120)/3

       A=29460/3

        A=9880

  vậy A là 9880

 lưa ý dấu này/ nghĩa là chia

Answer 2

3.A = 5.6.(7-4) + 6.7.(8-5) + ....+30.31.(32- 29)

3.A = 5.6.7 - 4.5.6 + 6.7.8 - 5.6.7 + ...+ 30.31.32 - 29.30.31

3.A = (5.6.7 + 6.7.8 + ...+ 30.31.32) - (4.5.6 + 5.6.7 + ...+ 29.30.31)

3.A = 30.31.32 - 4.5.6 = 29 640 => A = 9 880