a6＝a1×q^5
a7＝a2×q^5
a8＝a3×q^5
∴a6+a7+a8＝q^5﹙a1+a2+a3=3﹚
q^5＝﹙a6+a7+a8﹚/﹙a1+a2+a3﹚＝6÷3＝2
∴a11+a12+a13＝q^5﹙a6+a7+a8﹚＝2×6＝12