A contact of mine on Facebook kindly provided me with a set of harder problem style questions recently for one of my more capable students… These are questions where you can’t just apply the maths you know – You have to think a bit.
I’m not going to use them all for blog posts, but this was an interesting one I think.
First thought – How can we tell that? Your calculator isn’t going to help; Numbers that big are not going to give you the last digit.
So if we can’t tell directly, what can we tell? Actually, we need to start playing, and playing with numbers is something I love to do.
Let’s start by looking at what powers of 4 are like – That’s going to help us
You don’t have to go very far before seeing that every second number – every power of 4 to an odd number – end in a 4.
Continue that pattern on and we can see 4 to power 333 is going to end in 4.
What about powers of 3 – and here I did need my calculator – 3 9 27 81 243 729 2187 6561
Every fourth number in this list 4th, 8th – and indeed the one before the 3 would be 1, 3^ 0) ends in a 1.
All the numbers divisible by 4 in fact. So we can say for sure that 3 to the power 444 end in a 1.
And a number ending in a 4 plus a number ending in a 1 will end in a 5.
WE have shown what we were asked to show, and we didn’t have to work out the whole number