首项是a,公比是q
a5=aq^4=7.5+a
a(q^4-1)=7.5
S4=a(q^4-1)/(q-1)=7.5/(q-1)=-5
q-1=-1.5
q=-0.5
a(q^4-1)=7.5
a=7.5/(q^4-1)=7.5/(1/16-1)=7.5/(15/16)=8
所以a4=aq^3=8*(-1/2)^3=-1