//第k+1項的計算公式:a(k+1)=a(k)+a(k-1), 用括號代表下標
for(int i = 3; i<=20; i++ )
{
k = a1 + a2; //k代表第k+1項
s += k; //s代表累加總和
a1 = a2; //把第k項賦給a1
a2 = k; //把第k+1項賦給a2,下次計算k+2項
cout << k << endl; //打印出每壹項的數值
}