Question

已知数列an中,an=an-1+1/2,am=3/2,前m项和sm=-15/2,求a1和m

Answer 1

SM=(AM+A1)/X*M

=[2AM-1/2*(M-1)]*M

=(7/2-1/2*M)*M

整理后得

m^2-7m-30=0

(m-10)(m+3)=0

∴m=10

a1=1.5-9*0.5=-3