Preheat oven to 425°. Cut 3/4 pound sweet potatoes into 1/4-inch-thick slices; set aside. Combine 1 tablespoon olive oil, 1 1/2 teaspoons chili powder, 1/2 teaspoon salt, and 1/4 teaspoon ground cumin. Add sweet potatoes; toss gently to coat. Cover a lightly oiled nonstick baking sheet with a single layer of potatoes; roast, turning once, until golden and tender (about 20 minutes). Serve with store-bought light ranch dressing.