Assuming that b 12 is the arithmetic mean of b 11 and b 22 , b 12 = ( b 11 + b 22 ) / 2 , substituting into the quadratic form, and noting that x 1 + x 2 = 1 produces b = b 11 x 1 + b 22 x 2 Van der Waals wrote this relation, but did not make use of it initially. However, it has been used frequently in subsequent studies, and its use is said to produce good agreement with experimental results at high pressure.