1. How many sticks are needed to form Pattern 15?
2. In which pattern will each side of the triangle measure 45 cm?
3. Calculate the number of shaded triangles in Pattern 81.

(a)
Number of sticks:
Pattern 1: 1 x 3   = (1) x 3 = 3 sticks
Pattern 2: 3 x 3   = (1 + 2) x 3 = 9 sticks
Pattern 3: 6 x 3   = (1 + 2 + 3) x 3 = 18 sticks
Pattern 4: 10 x 3 = (1 + 2 + 3 + 4) x 3 = 30 sticks

Sum of numbers = Pattern Number x (Pattern Number + 1) ÷ 2

Formula:
Number of sticks = Sum of numbers up to Pattern Number x 3

Number of sticks needed for Pattern 15
= (1 + 2 + 3 + ... + 15) x 3
= [15 x (15 + 1) ÷ 2] x 3
= [15 x 16 ÷ 2] x 3
= 120 x 3
= 360

(b)
Length of each side of the triangle:
Pattern 1: 1 x 5 = 5 cm
Pattern 2: 2 x 5 = 10 cm
Pattern 3: 3 x 5 = 15 cm
Pattern 4: 4 x 5 = 20 cm

Formula:
Length of each side of the triangle = Pattern Number x 5

Pattern Number with each side of the triangle measuring 45 cm
= 45 ÷ 5
= 9 

(c)
Pattern 1: Number of shaded triangles = 0
Pattern 2: Number of shaded triangles = 0 + 1
Pattern 3: Number of shaded triangles = 0 + 1 + 2
Pattern 4: Number of shaded triangles = 0 + 1 + 2 + 3

Pattern for number of shaded triangles = Sum of numbers up to (Pattern Number - 1)

Number of shaded triangles in Pattern 81
= [(Pattern Number - 1) x (Pattern Number)] ÷ 5
= (80 x 81) ÷ 5
= 1296

Answer(s): (a) 360; (b) 9; (c) 1296