Question

Determinați numărul submultimilor cu doua elemente a mulțimii {1,2,3,4,5,6,7}

Answer 1

{1,2,3,4,5,6,7}

{1,1},  {1,2} , {1,3} , {1,4} , {1,5} , {1,6}, {1,7}, {2,2} {2,3}, {2,4}, (2,5}, {2,6}, {2,7},
{2,4}, {3, 3}, {3,4}, {3,5}, {3,6}, {3,7}, {4,4}, {4,5}, {4,6}, {4,7},  {5,5}, {5,6}, {5,7}
..............
A₇²=$A_{n}^{k}$=n!/(n-k)!=(1×2×3×4×5×6×7)/(7-2)!=(1×2×3×4×5×6×7)/(1×2×3×4×5)=6×7=42