循環寫法大家都會，我再補充兩個

//遞歸
public?int?cool(int?a)?{
????if?(a?<?0||a?%?2?==?0)?{
????????throw?new?RuntimeException("請輸入大于1的基數");
????}
????if?(a?==?1)?{
????????return?1;
????}
????return?a?+?cool(a?-?2);
}
//數學歸納法
public?int?cool2(int?a)?{
????if?(a?<?0||a?%?2?==?0)?{
????????throw?new?RuntimeException("請輸入大于1的基數");
????}
????return?a?*?a?/?4?+?a?/?2?+?1;
}

public?int?doCalculate()?{
????int?sum?=?0;
????int?i?=?1;
????while(i?<=?99)?{
????????sum?=?sum?+?i;
????????i?=?i?+?2;
????}
????return?sum;
}

public?static?void?main(String[]?args)?{
???int?sum?=0;
???for?(int?i=0;i<50;i++){
??????sum?=?sum?+?(2*i)+1;
???}
???System.out.println(sum);
}