In order to simplfy square roots of non perfect squares ( 16, 25, 36 are perfect squares ) the easiest thing to do is to break your number down into its prime factors.
Prime numbers are those numbers which only are divisable by 1 and itself. 5 is prime, because only 1 and 5 divide into 5 evenly. However, 9 is NOT prime because 3 also divides into 9.
So to prime factor 245, we need to find its prime factors, the prime factors of 245 are 7, 7, 5 we know this because 7*7*5 = 245.
A rule of radicals states that 
= 
* 
Now simplfy each square root, 
and can not be simplfied
Mulply the two together and you get 
is the same, the prime factors of 80 are (2*2*2*2*5)
Rewrite as 
Simply 
Now, as for the missing length of right triangles, you must use the pythagorean therom 
Plug in what you know 
Simplfy 
Simplfy 
so 
and use the process from above to simplfy 
For the last problem, I am assuming that A = 2 since a length of a triangle leg can not be negative,
The square of any square root is the number so we have
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