Yes there is. i do not know the theorem but I know it's rule. If 2 lines intersect the same line, and they have an equal angle in the same quadrant then they are parallel. This is because their all of their angles would be equal and they are basically the same line but shifted over
        
                    
             
        
        
        
7978 years
Step-by-step explanation:
A = A02^(-t/hl)
where hl = half-life. 
Dividing both sides by A0 and taking the logarithm, we get
ln(A/A0) = -(t/hl)ln2 
or solving for t,
t= -(hl)ln(A/A0)/ln2
note that A/A0 = 0.38
t = -(5715 yrs)[ln(0.38)/ln2]
 = 7978 years
 
        
             
        
        
        
Isosceles triangle has 2 sides that are the same length. So what you have is three sides that measure: x, x, and (x-8). 70 = x + x + (x-8).
70 = x + x + x - 8
78 = 3x
x = 26.
and 26 - 8 = 18.
So two of your sides are 26 inches and the last side is 18 inches. 
        
                    
             
        
        
        
For example, exc. 15.
x + y = 10 and 6/x = 9/y; You need y!
=> x = 10 -y and 9x = 6y => 9(10-y) = 6y => 90 - 9y = 6y => 15y = 90 => y = 6.