equations

x + a = xa

x + a + b = (x-a) + (x+b)

(x+b) + (x-a) = (x+b) + (x+a)

(x+b) + (x+a) = x + a

x + a = xa

xa = x/a, a/x, a-x, x-a, a, x

x - a = x + c

x + c = xc

xc = (xa + xd)/2 + xc/2