Describe the surface in 3 represented by the equation x + y = 9. This is the set {(x, 9 − x, z)|x , z } which is a vertical plane that intersects the xy-plane in the line y = 9 − x, z = 0. This is the set {(x, 9 − x, z)|x , z } which is a horizontal plane that intersects the xz-plane in the line y = 9 − x, z = 0. This is the set {(x, 9 − x, z)|x , z } which is a horizontal plane that intersects the xy-plane in the line y = 9 − x, z = 0. This is the set {(x, y, 9 − x − y)|x , y } which is a vertical plane that intersects the xy-plane in the line y = 9 − x, z = 0. This is the set {(x, 9 − x, z)|x , z } which is a vertical plane that intersects the xz-plane in the line y = 9 − x, z = 0.