A=25n+5n−18n−12n⎧⎩⎨=(25n−18n)−(12n−5n)⋮7=(25n−12n)−(18n−5n)⋮13→A⋮91
A=5n(5n+1)−6n(3n+2n)" role="presentation" style="border:0px; direction:ltr; display:inline; float:none; font-style:normal; line-height:normal; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax">A=5n(5n+1)−6n(3n+2n) chia hết cho 91 với mọi số tự nhiên n