Economics concepts

Law of demand

If the price of the product rises the quantity demand of that product goes down all other factor held constant.
Examples
1.When Starbucks reduces it's coffee price from Rs 200 to Rs 99 there was a huge demand. People were seen standing in big line .
2.many times we have seen when onion prices goes up the quantity demand for onion goes down

Law of supply

If the price goes up quantity supply also goes up other factor held constant
Examples

1.In summer air conditioner price goes up so the supplier supplies more quantity to the market
2.In festive season the price of fruits goes up and the supply also goes up

Opportunity cost

It refers to what you have to give up to buy what you want in terms of other goods and services.
Examples
1.I have applied to two colleges  IBA and ifim. I compared the two on the basis of rate of investment and placement and then selected iba. So opportunity cost of iba is ifim
2.Before purchase of a new phone I was confused between red mi and oppo after comparing the two on the basis of price features etc I purchased oppo so for me opportunity cost of oppo is redmi

Diminishing marginal utility

It states that as the consumer has more and more of a good the marginal utility that he derives goes on diminishing.
Examples
1.In the beginning when we joined iba the food that is given in hostel has a certain positive utility with each passing day the marginal utility from hostel food goes on diminishing
2.when I purchased my new phone the marginal utility I derive is much higher than today.

Complementary goods

Complementary goods are those goods which are jointly needed to satisfy human want.
Examples
1.Car and petrol
When petrol prices are low the demand for Petrol car rises now when petrol prices are all time high now consumer are looking for ecars
2.Tea and sugar
When there  is reduction in sugar prices the demand for tea increases.