等差数列题目求解！！！急！！！

显示全部楼层 · 2009-7-20 11:52:44

a2与a3平方的和是（a2）^2+(a3)^2吗？邮箱多少？设an=a+(n-1)dd不等于0 （a2）^2+(a3)^2=（a4）^2+(a5)^2即（a+d）^2+(a+2d)^2=（a+3d）^2+(a+4d)^2解得a=-5d/2再由Sn=n(a1+an)/2s7=7得d=2即an=2n-7令（（am）^2+1）/(am+2)=an即（（2m-7）^2+1）/(2m-5)=2n-7即(2m-7)^2=(2n-7)(2m-5)+1左边为奇数右边为偶数故这样的M不存在是Am与Am+1的积比上Am+2也一样利用奇偶性不同得出等式不成立m不存在帅哥难道是am...

千问 · 2009-7-20 11:52:44

a(n)=a+(n-1)d,S(n)=na+n(n-1)d/2.7=S(7)=7a+21d,1=a+3d,a=1-3d.[a(2)]^2 + [a(3)]^2 = [a+d]^2 + [a+2d]^2 = [a(4)]^2 + [a(5)]^2 = [a+3d]^2 + [a+4d]^2,2ad + d^2 + 4ad + 4d^2 = 6a...