Let R and the three vectors
a = k ˆ3j ˆi ˆ ,
b = k ˆ–j ˆi ˆ2 and
c = k ˆ3j ˆ2–i ˆ . Then the set
S = { :
a ,
b and
c are coplanar} (1) contains exactly two numbers only one of which is positive (2) is singleton (3) contains exactly two positive numbers
