3(a1+a3)/2+5(a1+a5)/2=503(2a1+2d)/2+5(2a1+4d)/2=508a1+13d=50a1a13=a4a4a1(a1+12d)=(a1+3d)^212a1d=9d^2a1=3/2d25d=50d=2a1=3an=3+2(n-1)=2n+1
