A3=A1+2d=5
A1=5-2d
A2XA4=(A1+d)(A1+3d)=(5-2d+d)(5-2d+3d)=21
(5-d)(5+d)=21
25-d^2=21
d^2=4
d=2 d=-2(舍去,因为各项均为正数)
所以A1=1
An=A1+(n-1)d=1+(n-1)*2=2n-1
A3=A1+2d=5
A1=5-2d
A2XA4=(A1+d)(A1+3d)=(5-2d+d)(5-2d+3d)=21
(5-d)(5+d)=21
25-d^2=21
d^2=4
d=2 d=-2(舍去,因为各项均为正数)
所以A1=1
An=A1+(n-1)d=1+(n-1)*2=2n-1