the above will yield a quadratic form of three variables. the quadratic form of the above is:
a11x1^2 + 0x2^2 + a33x3^2 + a12x1x2 + a13x1x3 + a23x2x3
i included x2^2 even though its coeffcient is zero, i don't think you need to do this, i just like being thorough.
see Quadratic form for more info, the first paragraph and illustration has everything you need
if you don't get the relationship get back to me