5 x 5/2 = 25/2
answer
25/2 units
<h3>Given</h3>
4 hundreds flats; 5 tens rods; 2 ones cubes
<h3>Find</h3>
The number of hundreds flats in each of 2 equal piles
<h3>Solution</h3>
When 4 flats are divided into two equal groups, each group will have ...
... 2 flats
_____
You can imagine doing this the way a card dealer might: first put 1 flat in each of 2 piles, then do the same for the remaining 2 flats. Each pile will end up with 2 flats.
— — — — —
You will have a problem if you continue with the tens rods. There is an odd number of those, so one of them will have to be exchanged for 10 ones cubes.
P(7,6), Q(1,6), R(4,2)
We have PQ parallel to the x axis. We'll call that the base,
b = 7 - 1 = 6
The altitude is then the y difference h = 6 - 2 = 4
The area is 
Answer: 12 square units
In general we can use the shoelace formula for the area of any polygon given coordinates. We write the points like this:
(7,6), (1,6), (4,2)
(1,6), (4,2), (7,6)
The area is then half the absolute value of the sum of the cross products:

I assume you happen to mean, what is the formula for a circle.
The formula is:
A=