楼上没做完啊a1+a2+a3+a4+a5=3,...① a1^2+a2^2+a3^2+a4^2+a5^2=12,...② 则a1-a2+a3-a4+a5=x...③ 设①式等比为q,则②式为q^2,③式为-q 由等比求和公式,①式可化为a1(1-q^5)/(1-q)=3 ②式可化为a1^2(1-q^10)/(1-q^2...