Question

cho x thuộc N* cho ba số tự nhiên liên tiếp tăng dần, giảm dần

Answer 1

S={0,1,2,3,4,5,6,7,8,9}" role="presentation" style="border:0px; direction:ltr; display:inline-block; float:none; font-size:14.04px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:1px 0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax_CHTML mjx-chtml">S={0,1,2,3,4,5,6,7,8,9}

N=a5a4a3a2a1¯" role="presentation" style="border:0px; direction:ltr; display:inline-block; float:none; font-size:14.04px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:1px 0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax_CHTML mjx-chtml">N=a5a4a3a2a1¯ với ai∈S, a5≠0 và a1+a2+a3+a4+a5= chẵn. Ta chỉ có 3 TH sau :

  TH1 - 5 chữ số chẵn_" role="presentation" style="border:0px; direction:ltr; display:inline-block; float:none; font-size:14.04px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:1px 0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax_CHTML mjx-chtml">  TH1 - 5 chữ số chẵn_ : có 4.5.5.5.5=4.54 cách

  TH2 - 3 chữ số chẵn và 2 chữ số lẻ_" role="presentation" style="border:0px; direction:ltr; display:inline-block; float:none; font-size:14.04px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:1px 0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax_CHTML mjx-chtml">  TH2 - 3 chữ số chẵn và 2 chữ số lẻ_ :

3" role="presentation" style="border:0px; direction:ltr; display:inline-block; float:none; font-size:14.04px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:1px 0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax_CHTML mjx-chtml">3 chữ số chẵn trong S, có C53 cách2" role="presentation" style="border:0px; direction:ltr; display:inline-block; float:none; font-size:14.04px; line-height:0; margin:0px; max-height:none; max-width:none; min-height:0px; min-width:0px; padding:1px 0px; position:relative; white-space:nowrap; word-spacing:normal; word-wrap:normal" class="MathJax_CHTML mjx-chtml">2 chữ số lẻ trong S, có C52 cách