∵a2a4=9(a3＋a4)∴a3的平方＝9（a3+a3q）∴a3的平方＝9a3(1+q)∴a3＝9(1+q)∴a1＝9(1+q)/q^2其所有项之和为S=a1/(1-q)所有偶数项之和为s偶＝a1q/(1-q^2)∴a1/(1-q) ＝4×a1q/(1-q^2)∵a1>0∴1/(1-q) ＝4×q/(1-q^2)...