共回答了1627个问题 举报
an =a1q^(n-1)
a1+a2=3
a1(1+q) =3 (1)
a3+a4=12
a1q^2(1+q)=12 (2)
(2)/(1)
q^2 = 4
q=2 or -2
case 1: q=2
a1= 1
an = 2^(n-1)
S5 = 2^5 -1 = 31
case 1: q=-2
a1= -3
an = -3(-2)^(n-1)
S5 = -( 1-(-2)^5) = -33
a1+a2=3
a1(1+q) =3 (1)
a3+a4=12
a1q^2(1+q)=12 (2)
(2)/(1)
q^2 = 4
q=2 or -2
case 1: q=2
a1= 1
an = 2^(n-1)
S5 = 2^5 -1 = 31
case 1: q=-2
a1= -3
an = -3(-2)^(n-1)
S5 = -( 1-(-2)^5) = -33
1年前
2
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