Hands-on math!

Washington State Math Olympiad
Hints and Solutions
2016 Grade 6 Geometry

Problem
Solution
1) A square can be split to make four smaller squares of equal sizes to completely fill the original square as shown. How would you split up the original square into six smaller squares that completely fill the original square without any overlaps of the smaller squares?
(The squares do not all need to be the same size.)
1. Cut the square into 9 equal sized smaller ones.
2. You should be able to solve this once you look at this diagram by combining these smaller squares.
The answer should look like this:
Any combination of 4 smaller squares to make a larger square are OK.

2) If the total surface area of a cube is 150 sq. cm., what is its volume?


1. The surface area consists of 6 of the faces.
2. Set this equal to 150 and solve for the length of a side: =
    6A = 150
    A = 25
    S (the length of a side = 5 cm.
3. Compute the volume as the side length cubed =
    53 = 125 cu. cm.
3) In rectangle ABCD, X is the midpoint of CD and Y is the midpoint of AD. What percentage of the figure does the shaded area represent?


1. Put the length of the sides XC, XD, AY and YD on the figure.
2. Compute the sum of the areas of the 3 non-shaded triangles:
    A1 = (6 x 8)/2 = 24
    A2 = (12 x 4)/2 = 24
    A3 = (4 x 6)/2 = 12
    A1 + A2 + A3 = 60
3. Subtract this from the area of the rectangle =
    (8 x 12) - 60 = 96 - 60 = 36
3. Divide this by the area of the rectangle =
    36/(8 x 12) = 6/16 = 3/8
4. Convert 3/8 to percent =
    3 / 8 = .375 x 100 = 37.5%

Problem
Solution
4) What is the sum of all the interior angles of an octagon?


Method 1: Use analysis
1. Put a dot in the center and draw lines from this dot to each vertex of the octagon. This makes 8 triangles with a common vertex in the center of the octagon.
2.Compute the center vertex angle of one triangle as 1/8 of a circle (because they all add to 360o) =
    360/8 = 45o
3. The other 2 angles of the triangle add to
    180 - 45 = 135o.
4. The interior angle of the octagon consists of 2 of these other angles, so the interior angle of a single octagon =
    135o
5. Multiply this by the number of vertices =
    8 x 135 = 1080o

Method 2: Use the interior angles of a polygon formula:
1. The sum of the angles of a polygon is 180 ( n - 2) where n = number of sides.
2. For n = 8:
    S = 180 x (8 - 2) = 180 x 6 = 1080o
5) How long is segment BC? 1. Draw a line from point c to line AB parallel to AD.
(See figure to the left)
2. This forms a triangle. Label it's sides from other information in the diagram.
    Side 1 = 12 ft.
    Side 2 = 30 - 14 = 16 ft.
At this point, you can solve it by 2 methods:
Method 1: Use the Pythagorean theorem
compute the length of BC =
    BC = √ (12 2 + 16 2 = √ 400  = 20 ft.

Method 2: Examine the triangle
1. The 2 given sides are 12 feet and 16 feet.
2. If you take out the common factor of 4 you get 3 feet and 4 feet.
3. Now it's just a 3-4-5 triangle!
    Put the common factor in the 5 ft. side and you get:
    5x4 = 20 feet