Help in factoring large integers

[Guest Blues_Boy]

I'm returning to school after 18yrs of hi-tek hell / BS - so lets be honest my in that my brain is still recovering from corporate insanity at the highest levels.

 

I'm taking bone head algebra (pre MTH95) and doing fine --- but I really freak out when it comes to factoring large numbers - esp during tests

 

A fine example is on the last test on square roots - I ran through a word problem getting the sq root of 6400 as the answer - and that need to be simplified - I freaked and blew it - obviously the sq root of 64 is 8 so 6400 would be 80 --- but that never made it thu my panic layer

 

 

What are good methods to break large numbers down beside guessing or divine intervention --- I've heard of factor trees etc ???? Any comments or web links highly appreciated

 

Finals in 2 weeks --- no pressure

[mcoy]

number trees is a very good idea, and probably the fastest that i know. try doing it using hex.

[ed84c]

Look in the libary for a book on calculating quickly. There was in our libary.

[uncool]

Just look for basic parts, like divisibility by 2, 3, 5, and 11.

Another thing to know: if a number is not divisible by any of the primes up to its square root, then it itself is prime.

-Uncool-

[The Thing]

Well, the most practical way is to cancel the zeros when you see them, but I don't recommend this as much, for, like doing sqr roots, 0s are very important. Sqr(1000) is not 10, so, yeah.

There are lots of tricks to calculating large numbers, such as getting the square of 2 digiter with ones digit of 5 instantaneously and stuff like that. Go find a book in the library and usually they have assortments of tricks.

[mcoy]

also try to memorize the basic identification techniques for small factorrs such as 3,7,8,11 etc. its easier that way to solve a lot of factorisation problems. tho you'll have to watch out for 2 or 3 digit prime numbers such as 13,17,23 etc. it might serve you useful.