1.Cho hinh chop S.ABCD day' la hinh binh hanh. Mp (P) cat SA,SB,SC,SD tai K,L,M,N. CMR:SASK+SCSM=SBSL+SDSN2.Cho tu dien S.ABC co SA=SB=SC=1 .Mat phang (P) di qua trong tam M cua tu dien, cat SA,SB,SC lan luot tai D,E,F khac S
a) CMR:→SM=14[1SD.→SD+1SE→SE+1SF→SF]
b) tim GTLN cua :T=1SD.SE+1SE.SF+1SF.SD