9. Using mathematical induction, prove that any decimal number, whose sum of digits is divisible by three, is itself divisible by three as well. (For example: for 1125 , the sum of digits is: 1+1+2+5=9, which is divisible by three. And, 1125 itself is also divisible by three: 3375.) a) What is a base case? b) What is the induction hypothesis? c) What is the induction step? d) How do you prove the induction step