Because there will be "seven, six, five, four, three ..." in the formula, it is easy to remember. Mathematical formula is an expression method that people find some connection between things in nature and express it in some way.

Mathematical formulas represent the equal or unequal relationship between the quantities of different things in nature, accurately reflect the internal and external relations of things, and are the basis for us to reach another thing from one thing, so as to better understand the essence and connotation of things.

There are only multiplication and division in Wallis formula, and even roots are not needed, so the form is very simple. Although Wallis formula has no direct influence on the approximate calculation of π, it plays an important role in the derivation of Stirling formula.

Huatouli's second formula:

∫(0→π/2)dt=∫(0→π/2)dt .

=(n- 1)！ ! /n！ ! (n is a positive odd number).

=π(n- 1)！ ! /(2(n！ ! (n is a positive even number).