 # Neat math tricks on youtube.

The terms used to explain some of it are poor metaphors but what they do is solid.

First is shown using binary (mixed with decimal but not) to multiply and divide and then the same only a different way. To multiply 17 by 25

## 1x1 25 2x0 50 4x0 100 8x0 200 16x1 400

= 25 + 400 = 425 <— then add the right column values that the left has 1’s not 0’s

17 x 25 = 1 x 25 + 16 x 25 = 425

Division goes the other way as they show.

I haven’t taken the time to proof the second method they show, need to work out the part of crossing out evens.

There are other fast math videos worth watching, algorithms to be gained!

Gypsy multiplication.

Hi, Where on youtube? Please read the sticky at the top of each forum list.... :) :)

17 x 25

17 x 100 = 1700 easy 17 x 50 = 850 easy 1/2 of 1700 17 x 25 = 425 easy 1/2 of 850

Thats how I'd do it, realizing 4 x25 =100 you only have to halve and halve, easy to do in your mind. (Two binary bit shifts) :) :)

Anything multiplied by 50 or 25.. easy.

But I appreciate the gymnastics of @GoForSmokes example.

Tom.... :)

Oops! Youtube video title is An Easy Math Trick Nobody Will Show You

The guy is a bit of a nut but the first part math technique itself is solid.

Any value times 25, you have 25 as your proportion and you can relate binary to doubling proportions.

There are other fast math videos, all kinds. Vedic maths look particularly interesting.

But did the Egyptians and Chinese really use binary? I don’t know of what he says but neither used zeros and this doubling proportions math adds what’s there with no need for what’s not, and doubling is easy.

Looked over the second trick in that video and got it.

You multiply two numbers by halving (right-shift) one and doubling (left-shift) the other and add the doubled column value to an accumulator when the halved column value is odd (& 1 == 1).

13 x 24 -- 13 is odd, total = 24 bit3 1 6 x 48 -- 6 is even, don't add bit2 0 3 x 96 -- 3 is odd, total += 96 bit1 1

# 1 x 192 1 is odd, total += 192 bit0 1

total = 312

1011 is binary 13 decimal, it's a high to low bit version of the first method shown, still only adds proportions for bits that add to 13. There is no 48 to add in 312 as there is no 4 in 0b1011, the low bit is 0.

I wonder how it compares with the multiply we have now? Must take a crack at that one and find a similar divide!

PS - I like how this one starts right at one value, is there a quick way to pick the operand with the least bits set to cut the number of adds? Maybe always halve the smaller value to cut steps!

Hi

13 x 24

13 x 25 13 x 100 = 1300 13 X 50 = 650 13 x 25 = 325

so 13 x 24 = 325 -13 = 312

Sorry, its 9:30am Sat morning and first coffee of the day has soaked in?

Don't ask me why 25 type solutions, keep popping into my head like that.

Could it be having to do it by mentally and by hand in High School, we had slide rules and the only electronic hand held calculator was owned by our Applied Maths teacher, a simple scientific Canon model, not sure what it cost, but must have been a pretty penny.

Tom... :)

As we was tooorrtt in sckool.

24

72

# +240

312

13 x 24??

10 x 24 = 240 3x24 = 72 240+72 = 312.

What's the problem?

I had a colleague who could multiply two 4 digit numbers in his head as a party trick.

Neat

Allan

# GoForSmoke: 13 x 24 – 13 is odd, total = 24 bit3 1 6 x 48 – 6 is even, don’t add bit2 0 3 x 96 – 3 is odd, total += 96 bit1 1 1 x 192 1 is odd, total += 192 bit0 1

total = 312

Slower than Arduino multiply for 32 bit values. MUCH slower.

``````#include "Arduino.h"

long bmult32( long smaller, long bigger ) // product must fit in long
{
long total = 0;
while( smaller > 0 )
{
if ( smaller & 1 )
{
total += bigger;
}
smaller >>= 1;
bigger <<= 1;
}
}

long product;
long bproduct;
long paritycount;
long over16count;

unsigned long startUsec;
unsigned long endUsec;
unsigned long elapsedUsec;

void setup()
{
Serial.begin( 115200 ); // if you can run faster with your IDE then do!
Serial.println( F( "\n\nStartup!\n" ));
Serial.print( F( "23 x 57 = " ));
product = bmult32( 23, 57 );
Serial.println( product );
Serial.print( F( "633 x 15767 = " ));
product = bmult32( 633, 15767 );
Serial.println( product );
Serial.print( F( "2100 x 17500 = " ));
product = bmult32( 2100, 17500 );
Serial.println( product );

startUsec = micros();
for ( long x = 2; x < 2000L; x++ )
{
for ( long y = x + 1; y < 3000L; y++ )
{
product = x * y;
if ( product & 1 )
{
paritycount++;
}
if ( product > 0xFF )
{
over16count;
}
}
}
endUsec = micros();
elapsedUsec = endUsec - startUsec;

Serial.print( F( "\ntime in micros = " ));
Serial.println( elapsedUsec );
Serial.print( F( "odd parity count = " ));
Serial.println( paritycount );
Serial.print( F( "over 16 bit count = " ));
Serial.println( over16count );

paritycount = over16count = 0;
startUsec = micros();
for ( long x = 2; x < 2000L; x++ )
{
for ( long y = x + 1; y < 3000L; y++ )
{
bproduct = bmult32( x, y );
if ( bproduct & 1 )
{
paritycount++;
}
if ( product > 0xFF )
{
over16count;
}
}
}
endUsec = micros();
elapsedUsec = endUsec - startUsec;

Serial.print( F( "\ntime in micros = " ));
Serial.println( elapsedUsec );
Serial.print( F( "odd parity count = " ));
Serial.println( paritycount );
Serial.print( F( "over 16 bit count = " ));
Serial.println( over16count );
Serial.println( );

for ( long x = 2; x < 2000; x++ )
{
for ( long y = x + 1; y < 3000; y++ )
{
product = x * y;
bproduct = bmult32( x, y );
if ( product != bproduct )
{
Serial.print( x);
Serial.print( F( " x " ));
Serial.print( y );
Serial.print( F( " ? " ));
Serial.println( product );
Serial.print( F( " != " ));
Serial.println( bproduct );
}
}
}

}

void loop()
{
}
``````

Startup!

23 x 57 = 1311

633 x 15767 = 9980511

2100 x 17500 = 36750000

time in micros = 10057268

odd parity count = 998001

over 16 bit count = 0

time in micros = 61617356

odd parity count = 998001

over 16 bit count = 0

AFAIK Babylonians grouped numbers by 5s and 10s like the Romans, and used a base-60 system. From whence there are 60 seconds in a minute, and 360 degrees in a circle's arc.

Yeah, the Babylonians used base 60. 60 is evenly divisible by 2,3,4,5,6,10,12,15,20 and 30. They had no decimals or 0.

The practice of halving one side of a multiply while doubling the other is shown old. Speculation about is is speculation.

Move coins on a table and these old ways show themselves.