{an}是d不等于0的等差数列,a2^2+a3^2=a4^2+a5^2,S7=7,求an

设an=a+(n-1)d d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)/2 s7=7,
S7=7a+1/2*7*(7-1)d=7
7*(-5/2d)+21d=7
得d=2,a=-5/2d=-5
即an=a+(n-1)d=-5+2(n-1)=2n-7