令cosx=t,t∈[-1,1]则f(t)=t²-3t+2对称轴为t=3/2∴f(t)在[-1,1]为减∴f(t)min=f(1)=0 ∵a5+a7=2a6=26∴a6=a1+5d=13又a3=a1+2d=7∴d=2,a1=3∴an=2n+1sn=(a1+an)n/2=(2n+4)n/2=n(n+2)=n²+2n bn=1/(an²-...