Dear Student,
Sperm (1N) + egg (1N) à Zygote (2N) ……….. Fertilization/Syngamy
2nd sperm (1N) + Polar nuclei (2N) à Endosperm (N) ……... Triple fusion
Now, during the course of development Endosperm is formed from the Primary Endosperm cell having primary endosperm nucleus which is a result of triple fusion.
Regards.