a1+a4+a7+……+a28
=a1(1+2^3+2^6+……+2^27)
=a1[1-(2³)^10]/(1-2³)
=100
a3+a6+a9+……+a30.
=a1(2^2+2^5+……+a^29)
=a1{2^2[1-(2³)^10]}/(1-2³)
=4a1[1-(2³)^10]/(1-2³)
=4x100
=400