Naughty Number Nine

Naughty Number Nine is a song in Schoolhouse Rock!, which can be seen in Mulitiplication Rock. The song teachs us how to multiply by 9.

Lyrics
''Number nine will put you on the spot. Number nine will tie you up, oh, in a knot. When you're tryin', Multiplyin' by nine, You might give it everything you've got And still be stopped. If you don't know some secret way you can check on, You'll break your neck on Naughty number nine.''

Now the first thing to keep in mind, When you're multiplyin' by nine Is that it's one less than ten. You see, nine is the same as ten minus one. So you could multiply your number by ten, And then subtract the number from the result, And you'd get the same product As if you'd multiplied by nine And you knew it.

I mean, eight times nine is 80 minus eight, And seven times nine is 70 minus seven, and six times nine is 60 minus six. You could use those tricks. 'Cause you must have some secret way you can beat it, Or else you'll meet it With naughty number nine.

Of course, it doesn't hurt To know the table of nines by memory. It goes like this: One times nine is nine, and two times nine is 18. (Mean ol' number nine) ''Three times nine is 27, and four times nine is 36. Five times nine is 45, and six times nine is 54, And seven times nine is 63. Eight times nine is 72, and nine times nine is 81, And ten times nine is 90.''

Now the digit sum is always equal to nine. I mean, if you add two and seven, the digits, You get nine, the digit sum. That's true of any product of nine. If they don't add up, you've made a mistake. 'Cause you must have some secret way you can check it, Or else you'll wreck it With naughty, nasty, mean old number nine.