Prove that if c is a positive real number and x is any real number, then -c </= x </= c if and only if |x|</= C.
</= greater than or equal to
Thus, by the definition of :
combining these two inequalities, we obtain:
now try to go the other way. i would probably try to reverse this exact proof. so split the inequality into two, and work on each case by case (if x > 0 or x < 0)