a1=a
a2=a+d
a3=a+2d
a4=a+3d
a5=a+4d
S3=3a+3d
S4=4a+6d
S5=5a+10d
1/3S3与1/4S4的等差中项为1
1/3S3+1/4S4=2
(1/3S3)(1/4S4)=(1/5S5)^2
∴(a+d)+(a+1.5d)=2
(a+d)*(a+1.5d)=(a+2d)^2 ,3a+5d=0
a=4,d=-12/5
等差数列{an}的通项公式:4-(n-1)*12/5=32/5-12n/5
a1=a
a2=a+d
a3=a+2d
a4=a+3d
a5=a+4d
S3=3a+3d
S4=4a+6d
S5=5a+10d
1/3S3与1/4S4的等差中项为1
1/3S3+1/4S4=2
(1/3S3)(1/4S4)=(1/5S5)^2
∴(a+d)+(a+1.5d)=2
(a+d)*(a+1.5d)=(a+2d)^2 ,3a+5d=0
a=4,d=-12/5
等差数列{an}的通项公式:4-(n-1)*12/5=32/5-12n/5