Many real-world situations can be modeled by direct variation. In geometry, the perimeter of a square is proportional to the side length of the square by the formula . The formula describes how distance is proportional to time by the constant of proportionality , which represents a constant rate of travel. When shopping for several same-price items, the total cost is proportional to the number of items by the price of each item, or .Sales tax is another example of direct variation. The amount of sales tax on an item is proportional to the price. If the sales tax rate is , then the amount of sales tax on an item with price is . The total cost of the item with tax is:
Determine two points on the line of the equation.The graph of a direct variation passes through the point and , where is the constant of proportionality. In this situation, is 1.1.