sqrt(20) = sqrt(4·5) = sqrt(4)·sqrt(5) = 2sqrt(5)
sqrt(45) = sqrt(9·5) = sqrt(9)·sqrt(5) = 3sqrt(5)
sqrt(80) = sqrt(16·5) = sqrt(16)·sqrt(5) = 4sqrt(5)
2·sqrt(125) = 2·sqrt(25·5) = 2·sqrt(25)·sqrt(5) = 2·5·sqrt(5) = 10sqrt(5)
1 / [sqrt(5) - 2] · [sqrt(5) + 2] / [sqrt(5) + 2] = [sqrt(5) + 2] / [5 - 4] = sqrt(5) + 2
sqrt(20) + sqrt(45) + sqrt(80) - 2·sqrt(125) + 1 / [sqrt(5) - 2]
= 2sqrt(5) + 3sqrt(5) + 4sqrt(5) - 10sqrt(5) + [ sqrt(5) + 2 ] = 2