Answer:
7.5
Step-by-step explanation:
= 0.5477225575
Multiply it by 5
= 2.738612788
Square it
7.5 
Hope this helped. :)
Answer:
B
Step-by-step explanation:
In A, it says that 9s +7c can be lower or equal to 160. This is wrong.
In C, it says that 9s +7c can only be higher than 160, when the question says at least, which means it can also be equal to 160, making it wrong.
In D, it says that 9s +7c can be lower or equal to 20. This is wrong.
Through elimination, only B can be right.
Answer:
hope this helps you out good luck
If it's a regular octahedron, then you can calculate it using the formula

where

is the length of the edge.
If it's not regular, you have to calculate the area of each side individually.