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Measurement and Geometry Review
Two similar cylinders have radii 3 cm and 6 cm. The smaller has volume 36$pi$ cm³. Find the larger volume without calculating from scratch.
Learning Intentions
Know
- Surface area
- Volume
- Scale factor
- Similarity
- Congruence
Understand
- How scale factors affect area and volume
- When to use which formula
Can Do
- Calculate SA and volume of prisms and cylinders
- Apply scale factors to area and volume
- Prove congruence and similarity
Area and Volume Formulas
Key formulas:
- Rectangular prism: $V = lwh$, $SA = 2(lw + lh + wh)$
- Cylinder: $V = pi r^2 h$, $SA = 2pi r^2 + 2pi r h$
- Any prism: $V = A_{ ext{cross}} imes h$
For composite solids: decompose, calculate each part, add/subtract.
Scale Factors
If lengths scale by factor $k$:
$ ext{Area} ightarrow k^2$
$ ext{Volume} ightarrow k^3$
Example: Scale factor 2. If original area = 10, new area = 40.
If original volume = 8, new volume = 64.
Congruence and Similarity
Congruence tests: SSS, SAS, ASA, RHS
Similarity tests: SSS (sides in proportion), SAS (two sides proportional, included angle equal), AA (two angles equal)
Remember: AAA proves similarity, NOT congruence.
Check Understanding
A model building is 1:50 scale. If the model volume is 0.4 m³, what is the actual volume?
Measurement Review
A cylinder has radius 4 cm, height 10 cm. Find volume and surface area.
$V = pi(4)^2(10) = 160pi approx 502.7$ cm³
$SA = 2pi(16) + 2pi(4)(10) = 32pi + 80pi = 112pi approx 351.9$ cm²
Two similar cones have heights 5 and 15. The smaller has volume $20pi$. Find the larger volume.
$k = 15/5 = 3$
$V_{ ext{large}} = 3^3 imes 20pi = 27 imes 20pi = 540pi$
Prove that two equilateral triangles with sides 4 and 8 are similar but not congruent.
All angles = 60°, so similar by AA.
Sides are in ratio 1:2, not equal, so not congruent.
Common Misconceptions
Apply linear scale factor to area. No — area scales as $k^2$. If lengths double, area quadruples.
Apply area scale factor to volume. No — volume scales as $k^3$. If lengths double, volume increases 8-fold.
Use AAA for congruence. AAA proves similarity only. Triangles with the same angles can be different sizes.
Practice — Measurement Review
Architecture and Engineering
The Sydney Opera House required extensive scale modelling before construction. Architects calculated how surface area and volume would scale from model to full size to estimate materials, costs, and structural requirements.
📓 Copy Into Your Books
▼Formulas
- Cylinder: $V=pi r^2 h$
- Prism: $V=A_{ ext{cross}} imes h$
- SA = sum of all faces
Scale
- Length: $k$
- Area: $k^2$
- Volume: $k^3$
Tests
- Congruence: SSS, SAS, ASA, RHS
- Similarity: SSS, SAS, AA
- AAA ≠ congruence