COMPLEMENTARY = 90° (C for Corner!) · SUPPLEMENTARY = 180° (S for Straight!)
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TRIANGLE ANGLESMOST MISSED ★
🔺 The 180° Rule
A triangle has angles of 55° and 72°.
What is the third angle?
ALL triangles: angle1 + angle2 + angle3 = 180° (ALWAYS). Subtract what you know!
Don't forget: this is only for TRIANGLES (3 sides). Quadrilaterals add up to 360°, not 180°!
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PYTHAGOREAN THEOREMCRITICAL
📐 a² + b² = c²
A right triangle has legs a = 6 and b = 8.
Find the hypotenuse c.
c² = a² + b² → c² = 36+64 = 100 → c = √100 · c is ALWAYS the HYPOTENUSE (longest side, opposite right angle)
The hypotenuse is ALWAYS c (the longest side). You can't put a leg as c! Also √100 = 10, not 50.
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COMPLEMENTARY ANGLES
📐 Corner = 90°
Two angles are complementary.
One angle is 34°. What is the other angle?
Also: two angles are supplementary. One is 110°. Find the other.
COMPLEMENTARY = add to 90° (think: C = Corner of a room) · SUPPLEMENTARY = add to 180° (think: S = Straight line)
Easy to mix these up! C before S in the alphabet → 90° before 180°. Memory word: "C omes before S, 90° comes before 180°"
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AREA OF SHAPESFORMULA CONFUSION
📏 Area Formulas
Find the area of each shape:
A) Rectangle: length = 12, width = 7
B) Triangle: base = 10, height = 6
C) Circle: radius = 5 (use π ≈ 3.14)
Triangle = ½ × b × h (HALF because a triangle is half a rectangle!). Don't forget the ½!
Triangle area: you need the HEIGHT (perpendicular), not the slant side. b×h gives a rectangle area — divide by 2!
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VERTICAL ANGLESLOOK-ALIKE TRAP
✖️ X Marks the Equal Angles
Two lines intersect. One angle is 65°.
Find all four angles at the intersection.
VERTICAL ANGLES are EQUAL (opposite each other in the X shape) · Adjacent angles are SUPPLEMENTARY (add to 180°)
There are only 2 different angle values — opposite angles match! So: 65°, 115°, 65°, 115°
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PARALLEL LINES + TRANSVERSALMOST MISSED ★
🛤️ The Angle Pairs
Two parallel lines are cut by a transversal. One angle is 120°.
Find the alternate interior angle and the co-interior (same-side) angle.
ALTERNATE INTERIOR = EQUAL (Z-shape!) · CO-INTERIOR = add to 180° (C/U-shape!) · Arrows mean PARALLEL!
Co-interior angles are NOT equal! They add up to 180° (they look like they should be equal but they're not!)
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PERIMETER
🔲 Around the Outside
Find the perimeter of a rectangle with length = 15 cm and width = 9 cm.
A square has perimeter = 48 cm. What is the side length?
PERIMETER = add up ALL sides · Rectangle has 2 lengths + 2 widths = 2(l+w) · Square: all 4 sides are EQUAL
Perimeter ≠ Area! Perimeter = distance AROUND the shape (like a fence). Area = space INSIDE (like a floor).
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TYPES OF TRIANGLESCLASSIFICATION
🔺 Name That Triangle!
Classify each triangle by its angles AND its sides:
A) Angles: 60°, 60°, 60°
B) Angles: 90°, 45°, 45° — sides: 5, 5, 7.07
C) Angles: 30°, 60°, 90° — sides: 3, 4, 5
BY ANGLES: Acute (all <90°) | Right (one =90°) | Obtuse (one >90°) · BY SIDES: Equilateral (3 equal) | Isosceles (2 equal) | Scalene (none equal)
Which congruence rule applies: SSS, SAS, ASA, or AAS?
SAS = Side-Angle-Side (the angle is BETWEEN the two sides). Here: two sides AND the included angle are equal → SAS!
SSA (side-side-angle) is NOT a valid rule! The angle must be BETWEEN the two sides (SAS) to guarantee congruence.
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COORDINATE GEOMETRYMOST MISSED ★
📍 Distance Between Points
Find the distance between points P(1, 2) and Q(5, 5).
d = √[(x₂−x₁)² + (y₂−y₁)²]
Also find the midpoint of PQ.
Distance: use Pythagorean theorem on the coordinate grid! Midpoint = ((x₁+x₂)/2, (y₁+y₂)/2) — average the x's, average the y's
Don't subtract the coordinates in the wrong order — (x₂−x₁)² = (x₁−x₂)² either way (squaring removes the negative). But be careful with midpoint: ADD then divide, don't subtract!
🔐 Answer Key
Try all problems first before looking! Flip the page! 📖