Algebra · JSS 2, SS 1
Direct variation
When one quantity doubles, the other doubles too.
- y ∝ x means y = kx
What the letters mean
- y, x
- the two quantities
- k
- the constant of variation
Worked example
y varies directly as x, and y = 12 when x = 3. Find y when x = 5.
Step 1: Find k
12 = k × 3, so k = 4
Step 2: Use y = 4x
y = 4 × 5 = 20
Final answer: y = 20
Always find k first, then write the full equation.
See it
Play with this picture from the Direct and Inverse Variation lesson.
Gradient
2
rise ÷ run = 4 ÷ 2
Distance AB
2√5
≈ 4.47
Midpoint M
(1, 2)
average of A and B
Drag A and B (or focus one and use the arrow keys).