Brainliest? Get this correct What is the sum of the rational expressions below?

Ineed to learn how to solve this problem

Television sizes are based on the length of the diagonal of the television's rectangular screen. so, for example, a 70-inch television is a television having a diagonal screen length of 70 inches. if a 70 inch television's diagonal forms a 29°angle with the base of the screen, what is the vertical height (1) of the screen to the nearest inch? 70 inhin

Which geometric principle is used to justify the construction below?