Prove that the number 2+3+5+7+11+13+17 is irrational.
It is known that a>1. Is it always true that ⌊⌊a⌋⌋=⌊4a⌋?
It is known that a=x+y+xy, b=y+z+yz, c=x+z+xz. where x>0, y>0, z>0. Prove that a+b+ab>c.