Let Y1,...,Yn be random variables. Let Q1 and Q2 be quadratic form of
Y1,...,Yn. Show that a linear combination of Q1 and Q2 is also a quadratic
form. That is, show that
Q = c1 Q1 + c2 Q2
is a quadratic form of Y1,....,Yn.

I know that Q=c1Q1+c2Q2=Y'ciAiY but thats nto enough to prove it?