$f$ is a trigonometric function of the form $f(x)=a\cos(bx+c)+d$. Below is the graph of $f(x)$. The function has a maximum point at $(\pi,9)$ and a minimum point at $\left(\dfrac{3\pi}{4},1\right)$. **Find a formula for $f(x)$. Give an exact expression.** $\qquad f(x)=$ [[☃ expression 1]] $\qquad$