Joint variation is when one quantity varies proportionally with two or more other quantities. Some key points: - Joint variation can be represented by an equation of the form y = kxz, where y varies jointly as x and z with k as the constant of variation. - Examples include the cost of pencils varying jointly with the number bought, and the area of a triangle varying jointly with its base and height. - To solve a joint variation problem, the equation is set up and the constant of variation is determined using given values to find the value of the variable for a new set of quantities. - Joint variation has many real world applications like the force on an object varying jointly with its