等比数列求和公式推导
方法1：第一项a1公比q
a1a1a2a1qa3a1q2a4a1q3a
a1q
1a
1a1q
S
仁a1a1qa1q2a1q3a1q
1a1q
S
仁a1qa1qa1q2a1q3a1q
1
S
a1q
a1qS
S
qS
a1a1q
S
a11q
1q
方法2：等比数列求和公式推导1S
a1a2a3a
公比为q2qS
a1qa2qa3qa
q
a2a3a4a
1
3S
qS
a1a
141qS
a1a1qA
5S
a1a1qA
1q6S
a1a
q1q
f7S
a11q5