氢汽水
春芽
共回答了13个问题采纳率:84.6% 举报
1
a3=a2q,a4=a2q^2 a3^2=a2a4
a2a4=4,a2+a4=20/3
x^2-20x/3+4=0
(x-10/3)^2=100/9-4
x=10/3±8/3,a2=6,a4=2/3或 a2=2/3,a4=6
a2=6,a3=2,q=2/6=1/3 a1=a2/q=18,an=18*(1/3)^(n-1)
a2=2/3,a3=2,q=2/(2/3)=3,a1=a2/q=2/3/3=2/9 an=(2/9)3^(n-1)
2
a2=a1q=5,a5=a1q^4=1/4,a5/a2=q^3=1/4/5=1/20,q=(1/20)^(1/3)
3
-1+d=a1,-1+2d=a2,-1+3d=-4
-4-(-1)=3d,d=-5/3 a1=-1+d=-8/3,a2=-1+2d=-13/3
-1,b1=-1q,b2=-1q^2,b3=-1q^3,-4=-1q^4 -4/-1=q^4=4,q=2^(1/2),b2=-1q^2=-2
a2-a1/b2=-13/3-(-8/3)/(-2)=-13/3-4/3=-17/3
1年前
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