If [math]\frac{1}{a} + \frac{1}{b} + \frac{1}{c} =\frac{1}{a+b+c}[/math] where [math]a+b+c \neq 0, abc \neq 0,[/math] what is the value of [math](a+b) (b+c) (c+a)?[/math]
How do I prove that A ∆ B = (A ∪ B) \ (A ∩ B)?