a2 * a6 = (a1q ) * (a1q^5) = a1^2 * q^6 = -2 * 10 = -20；
故 a3 * a5 = (a1q^2 ) * (a1q^4) = a1^2 * q^6 = a2 * a6 = -20 。
等比数列{a(n)}满足关系
a(m+p)*a(m-p)=a²(m)
a(p)a(q)=a(p+m)*a(q-m)
不管用哪一个关系都有
a(3)*a(5)=a(2)*a(6)=-2*10=-20
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