Generally speaking, the comprehensive formula is calculated one by one, which is also called off-type calculation and recursive equation calculation.

For example: 1+2=3 3*3=9 9+6= 15.

( 1+ 2)*3+6

= 3*3+6

= 9+6

= 15

The operation at the same level refers to the same type of operation in mathematics, such as addition, subtraction, multiplication and division, multiplication and division, and square root.