Next, the square of the length of the left =c vector can be different from the length of the right C vector by about one, and the length of the a+b vector is known as the root number 2 (diagonal of the square) by the parallelogram rule, so the original formula is: the length of the C vector is equal to twice the root number (cosine value of the included angle between the C vector and the a+b vector), and the cosine value is the maximum value 1 when the included angle is 0 degrees.