Friday, August 03, 2007
Multiplying numbers between 11 and 19
Multiplying numbers between 11 and 19............
Now, to do this in your head, we are going to use a similar technique to that in (a) above. As long as you know your times tables and can add some simple sums you'll be able to do this - with practice! First let's try 12x13. 12-10=2 and 13-10=3 (i.e. just take the units digits). Either add 13+2 or 12+3 = 15 (this is the tens digit = 150). Multiply 2x3 = 6 (this is the units). Add 150+6 = 156 is the answer. Let's try 17x18. Add 17+8 or 18+7 = 25. Multiply 8x7 = 56. Add 250+56 = 306 is the answer. How about squaring numbers between 13 and 19...easy! Let's do 16 squared. 16+6=22, 6x6=36, then 220+36 = 256 is the answer!
P.S. Once you have mastered doing this in you head, you will then be able to multiply any numbers between 0 and 20 in your head. But what about single digit (2..9) multiplied by double digit numbers? Easy. You can simply use long multiplication! Here's how. Try 8x19 = 8x9 + 80 = 152. Try 6x14 = 6x4 + 60 = 84.
Now, to do this in your head, we are going to use a similar technique to that in (a) above. As long as you know your times tables and can add some simple sums you'll be able to do this - with practice! First let's try 12x13. 12-10=2 and 13-10=3 (i.e. just take the units digits). Either add 13+2 or 12+3 = 15 (this is the tens digit = 150). Multiply 2x3 = 6 (this is the units). Add 150+6 = 156 is the answer. Let's try 17x18. Add 17+8 or 18+7 = 25. Multiply 8x7 = 56. Add 250+56 = 306 is the answer. How about squaring numbers between 13 and 19...easy! Let's do 16 squared. 16+6=22, 6x6=36, then 220+36 = 256 is the answer!
P.S. Once you have mastered doing this in you head, you will then be able to multiply any numbers between 0 and 20 in your head. But what about single digit (2..9) multiplied by double digit numbers? Easy. You can simply use long multiplication! Here's how. Try 8x19 = 8x9 + 80 = 152. Try 6x14 = 6x4 + 60 = 84.
// posted by Kritidas @ 2:33 AM 
