12. What is the value of $\sqrt{20212020 \times 20202021-20212021 \times 20202020}$ ?
SolUTION $\square$
Let $y=20202020$. We may express the quantity under the square-root as $(y+10000) \times(y+1)-(y+10001) \times y=y^{2}+10001 y+10000-y^{2}-10001 y=10000$. The square root of 10000 is 100 .