[tex]\bold{\text{Answer:}\quad y=\dfrac{5}{2}\cos \bigg(\dfrac{\pi}{6}x\bigg)+\dfrac{13}{2}}[/tex]
Step-by-step explanation:
Use the formula y = A cos (Bx - C) + D Â Â where
- A = amplitude
- Period = 2Ï€/B
- Phase Shift = C/B
- D = vertical shift (aka midline)
Given: Max = 9, Min = 4, (1/2)Period = 6 → Period = 12
Amplitude (A) = (Max - Min)/2
           = (9 - 4)/2
           = 5/2
          Â
Midline (D) = (Max + Min)/2 Â
         = (9 + 4)/2
         = 13/2        Â
Period = 2Ï€/B Â Â
→ B = 2π/Period
   = 2π/12
   = π/6
Notice that the Maximum touches the y-axis so there is no phase shift and no reflection → C-value = 0 & A-value is positive
Now, let's put it all together:
A = 5/2, B = π/6, C = 0, D = 13/2
[tex]\large\boxed{y=\dfrac{5}{2}\cos \bigg(\dfrac{\pi}{6}x\bigg)+\dfrac{13}{2}}[/tex]
Note that your graph will NOT fit the graph given because the max occurs in January (x = 0) and the min occurs in July (x = 6). The graph provided has the min at x = 0 and the max at x = 6.Â