Law of demand states that demand and price has inverse relationship I.e, as price increases demand will fall down and vice versa , this shows that demand is dependent on price .

Demand curve slopes downward representing inverse relationship

There are many causes of demand curve being sloping downward but law of diminishing marginal utility is one of them.

Law of diminishing marginal utility ( MU)states that with each increase in quantity of output consumed the utility derived from it falls down .

1. If MU of a good is constant : this can be the case of some necessary goods when due to change in its prices the consumption remains same , hence demand curve will be vertical straight line showing change in only price but quantity consumed remains same.

2. Positive MU: it can be the case of inferior goods because in these goods as price decreases the demand for inferior goods also decreases and consumer would like to purchase better quality goods , resulting in positive MU , so thedemand curve slopes upward for inferior goods....