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Proportion with algebra
When y is proportional to a power of x, write a formula with a constant k, find k, then use it.
Direct proportion
y ∝ x means y = kx.
y ∝ x2 means y = kx2.
y ∝ √x means y = k√x.
y ∝ x2 means y = kx2.
y ∝ √x means y = k√x.
Inverse proportion
y ∝ 1x means y = kx.
y ∝ 1x2 means y = kx2.
y ∝ 1x2 means y = kx2.
Method
1. Write the formula with k.
2. Substitute the given pair to find k.
3. Write the formula with the value of k.
4. Use it to answer the question.
2. Substitute the given pair to find k.
3. Write the formula with the value of k.
4. Use it to answer the question.
y is directly proportional to x2. When x = 3, y = 36. Find y when x = 5.
- y = kx2
- 36 = 9k, so k = 4
- y = 4 × 25 = 100
Answer: 100
Write y = kxn or y = k/xn, use the given pair to find k, then substitute. ∝ means is proportional to.
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