当|q|>1时,sn=a1(1-q^n)/(1-q)
s4=5s2 ==>1-q^4=5(1-q^2)
求出q^2=4
a3=a1q^2=4a1=2 求出a1=1/2
a5+a7=a1q^4+a1q^6=8+32=40