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Advanced Problem Solving Module 9

Proof by induction is a really useful way of proving results about the natural numbers. If you haven't met this powerful technique before, this module will introduce you to the idea and method of induction. If you're already familiar, check out some of the problems and STEP questions that can be answered in this way!
Dirisibly Yours
problem

Dirisibly yours

Age
16 to 18
Challenge level
filled star empty star empty star
Find and explain a short and neat proof that 5^(2n+1) + 11^(2n+1) + 17^(2n+1) is divisible by 33 for every non negative integer n.
Tens
problem
Favourite

Tens

Age
16 to 18
Challenge level
filled star filled star empty star
When is $7^n + 3^n$ a multiple of 10? Can you prove the result by two different methods?