20. Identify the values of the variable for which each expression is defined where necessary, then expand and simplify. a) (3-5)(3+5) b) (4x+(x+) 21. Rationalize the denominator. a) 23-4 47 22. Rationalize the denominator. a) 57 3+5 b) 42+23 5 b) 25-4 73-7