Use of Variables in Common Rule

Explain the Law of Variable Proportions. Use diagram.

Find the rule which gives the number of matchsticks required to make the following matchstick patterns. Use a variable to write the rule. A pattern of letter T as T

Find the rule which gives the number of matchsticks required to make the following matchstick patterns. Use a variable to write the rule. A pattern of letter Z as Z

Find the rule which gives the number of matchsticks required to make the following matchstick patterns. Use a variable to write the rule. A pattern of letter U as U

What is the principle used in common balance?