已知等差数列{An}s4=14 s10+s7=30 求s9
s4=1/2{a1+a1+3d)=14,2a1+3d=28
s10+s7=30,1/2{a1+a1+9d)+1/2{a1+a1+6d)=30,4a1+15d=60
d=4/9,a1=40/3,
s9=1/2)(a1+a1+8d)=136/9