  We prove a general quadratic formula for basic hypergeometric series, from
which simple proofs of several recent determinant and Pfaffian formulas are
obtained. A special case of the quadratic formula is actually related to a Gram
determinant formula for Askey-Wilson polynomials. We also show how to derive a
recent double-sum formula for the moments of Askey-Wilson polynomials from
Newton's interpolation formula.
