Mrs.D Math · 8th Grade

DIRECT
HIT

Direct Variation · Slope & Y-Intercept
🔗 Proportional
📝 y = kx
🎯 Direct Variation
📈 y = kx
🎯 Through (0, 0)
🔮 Predict: y = 4(10) = 40
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📚 How It Works

What Is a Direct Variation?

y = kx  —  a proportional relationship

k is the constant of variation. The graph is a straight line through the origin (0, 0).

1
Graph test: straight line AND passes through (0, 0) → direct variation!
Crosses the y-axis anywhere else? NOT direct variation.
2
Table test: includes the point (0, 0) and every y ÷ x gives the same k.
(1, 4), (2, 8), (3, 12) → k = 4 every time ✓

Making a Prediction

Find k, write y = kx, then plug in the value you know!

✓
A car uses 4 oz of oil per gallon of gas → y = 4x
✓
How much oil for 10 gallons? → y = 4(10) = 40 oz
✓
Know y instead? Divide! y = 6x and y = 138 → x = 138 ÷ 6 = 23

How to Play

🔍
Is It DV? — Study the graph or table, then decide: direct variation or not?
🔮
Make a Prediction — Use y = kx to find the missing value. Type your answer.
🎰
Mixed — Both types together for the ultimate challenge!
🧮
Calculator — Tap the gold calculator button (bottom-left) any time. Try it in your head first, then verify!
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IS IT DV?
📺 Mrs.D Math Lesson
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