`\sqrt{2021} - \sqrt{(\sqrt{2020} - \sqrt{2021})^2}`
`= \sqrt{2021} - |\sqrt{2020} - \sqrt{2021}|`
`Do 2021 > 2020 `
`⇒ \sqrt{2021} > \sqrt{2020}`
`⇒ \sqrt{2021} - (\sqrt{2021} - \sqrt{2020}) `
`(Do \sqrt{2020} - \sqrt{2021} < 0)`
`⇒ \sqrt{2021} - \sqrt{2021} + \sqrt{2020}`
`= \sqrt{2020}`
