According to one of the signs of a parallelogram, the sum of the squares of its diagonals is equal to the sum of the squares of its sides, i.e.:
AC ^ 2 + BD ^ 2 = AB2 ^ + BC ^ 2 + CD ^ 2 + AD ^ 2.
Knowing also that the opposite sides of the parallelogram are equal (AB = CD), we write this equality with all known values:
6 ^ 2 + 2 ^ 2 = 2 * (sqrt11) ^ 2 + 2 * BC;
36 + 4 = 22 + 2 * BC;
40 = 22 + 2 * BC;
2 * BC = 18;
BC = 9cm.
Answer: BC = 9cm.
One of the components of a person's success in our time is receiving modern high-quality education, mastering the knowledge, skills and abilities necessary for life in society. A person today needs to study almost all his life, mastering everything new and new, acquiring the necessary professional qualities.