this is the main equation of the apt.
equation and properties 1 and 2 are statement about 4 vectors :x,b,1 and t.
property 1 states that x is orthogonal to 1 property 2 asserts that x id orthogonal to b . together these statements imply a geometric configuration that we can easily visualize if we fix the number of risky assets at N=2 ,which implies that all vectors have dimension 2. this is illustrated in figure 13.1