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Question

Suppose that C is directly proportional to the square of the distance d. C=81 wen d=3. Find the coefficient K and write the formula for C. Use your formula to find C when d=5.

Answer 1

Because C is directly proportional to the square of the distance d, we can write

C = Kd².

Let's use formula when d = 3.

We will have

81 = K·3² or 9K = 81

So K = 9.

Now we cam write formula for C. It is

C = 9d²

If we use formula for d = 5, we will get

C = 9·5² = 9 x 25 = 225