SOLUTION: For the following problem, y varies inversely as the square of x. If y = 81 when x = 9, find y when x is 3

Algebra ->  Equations -> SOLUTION: For the following problem, y varies inversely as the square of x. If y = 81 when x = 9, find y when x is 3      Log On


   

Question 1056249: For the following problem, y varies inversely as the square of x.
If y = 81 when x = 9, find y when x is 3
Answer by Edwin McCravy(20060) About Me  (Show Source):
You can put this solution on YOUR website!
y varies inversely as the square of x.
y%22%22=%22%22k%2Fx%5E2

If y = 81 when x = 9, 
Substitute that in

81%22%22=%22%22k%2F9%5E2

Solve for k

81%22%22=%22%22k%2F81

Multiply both sides by 81

6561%22%22=%22%22k

Substitute that in 

y%22%22=%22%22k%2Fx%5E2

y%22%22=%22%226561%2Fx%5E2  <-- that's the formula

find y when x is 3
Substitute 3 for x

y%22%22=%22%226561%2F3%5E2

y%22%22=%22%226561%2F9

y%22%22=%22%22729

Edwin