the square of the sum of x and y is less than 20

[math_celebrity]

the square of the sum of x and y is less than 20

The sum of x and y means we add y to x:
x + y

the square of the sum of x and y means we raise the term x + y to the 2nd power:
(x + y)^2

The phrase is less than means an inequality, so we write this as follows:
(x + y)^2 < 20