Quarter circle: θ=90∘\theta = 90^\circθ=90∘ → area =πr2/4= \pi r^2/4=πr2/4. Semicircle: θ=180∘\theta = 180^\circθ=180∘ → area =πr2/2= \pi r^2/2=πr2/2. These simplify quickly and appear in many combined-figure problems.