This page about Casting Out Nines explains a fairly simple trick for checking the correctness of addition. It’s couched as way more amazing than it actually is.
Buried deep within the pages of a very old book*, one of the members of themathlab.com’s research team found an AMAZING thing. It’s a method for checking math computation that is so easy, so elegant, that it evoked a VIOLENT emotional response within her.
This is how she felt.
Yes, we said VIOLENT. In plain truth, she ran screaming wildly down the hall ENRAGED that she had not been taught this little gem in all her years of school! She hadn’t even learned it in her six years of college majoring in mathematics.
She screamed at all of us,”This is UNBELIEVABLY CRIMINAL! How can I have made it through all those years of school and NEVER have been shown this? I’ve wasted MILLIONS of minutes of my life checking math problems by hand or with a calculator, when I could have done this in SECONDS!”
See what I mean? Basically, it’s a way to add up many-digit numbers and see if you got the right answer. And . . . well, look at the title of my post.