1.a.Ta có: {U9=U1+8dU2=U1+dU13=U1+12dU6=U1+5dSuy ra: {U1+8d=5(U1+d)U1+12d=2(U1+5d)+5⇔{U1=3d=4Từ đó: U20=U1+19d=79;U30=U1+29d=119
1.a.Ta có:
{U9=U1+8dU2=U1+dU13=U1+12dU6=U1+5dSuy ra:
{U1+8d=5(U1+d)U1+12d=2(U1+5d)+5⇔{U1=3d=4Từ đó:
U20=U1+19d=79;U30=U1+29d=119Suy ra: S30=302(U1+U30)=1830