At first, a rectangle had four corners. Now, if you want to add an angle to make it five, you need to make one angle into two. As shown in the above picture.

Extended data:

Parallelogram and rectangle, diamond and square;

For parallelogram, the unique properties of rectangle are: all four corners are right angles; The two diagonals are equal and equally divided (the basis for judging that the median line on the hypotenuse of a right triangle is equal to half of the hypotenuse). The unique nature of diamonds: all four sides are equal; Two diagonal lines are perpendicular to each other, and each diagonal line bisects a set of diagonal lines. The sum of the unique properties of rectangle and rhombus is the unique properties of square for parallelogram.

Generally speaking, to prove that a quadrilateral is a rectangle or a diamond, we must first prove that a quadrilateral is a parallelogram, and then prove that a parallelogram is a rectangle or a diamond. When proving whether it is a square, we can start from two aspects. Like proving a rectangle or a diamond, we first prove that it is a parallelogram, then prove that it is a rectangle or a diamond, and finally prove that it is a square through known conditions or verification.