To find the estimated area using an upper sum with four rectangles of equal width, you can use the formula:
a) Area≈∑⁴ᵢ=₁f(xᵢ)Δx
b) Area≈∑⁴ᵢ=₁f(xᵢ)Δx
c) Area≈4∑⁴ᵢ=₁f(xᵢ)Δx
d) Area≈ 1/2 ∑⁴ᵢ=₁f(xᵢ)Δx