'''5. Explain why the shape of an indifference curve for two goods that are perfect substitutes is a straight line going from the upper left down to the lower right. Extra credit: why must its slope be negative 1? | '''5. Explain why the shape of an indifference curve for two goods that are perfect substitutes is a straight line going from the upper left down to the lower right. Extra credit: why must its slope be negative 1? |