1) \(36\times28+64\times69+4\times83\times9+8\times41\times8\)
\(=36\times 28+4\times9\times83+64\times69+8\times8\times41\)
\(=36\times 28+36\times83+64\times69+64\times41\)
\(=36\times(28+83)+64\times(69+41)\)
\(=36\times111+64\times110\)
\(=36\times(110+1)+7040\)
\(=36\times110+36\times1+7040\)
\(=3960+36+7040\)
\(=(3960+7040)+36\)
\(=11000+36=11036\)
2) Gọi số có 3 chữ số đó là \(\overline{abc}\) ta có:
\(\overline{abc12} = \overline{abc} + 53769\)
\(\overline{ abc}\times 100 + 12 =\overline{ abc}+53769\)
\(\overline{ abc} \times 99 + 12 =53769 \)
\(\overline{ abc} = (53769 - 12): 99\)
\(\overline{ abc} = 543\)
3) \(1+2+3+...+97+98+99\)
\(=(\dfrac{99-1}{1}+1)\times\dfrac{99+1}{2}\)
\(=99\times50=(100-1)\times50=100\times50-1\times50=5000-50=4950\)
