`\qquad 2/3sqrt{81}-1/2sqrt{16}`
`=sqrt{4/9. 81}-sqrt{1/4. 16}`
`=sqrt{36}-sqrt{4}`
`=6-2`
`=4`
--------------------------------
`\qquad 2/5sqrt{25/16}-1/2sqrt{4/9}`
`=2/5.sqrt{(5/4)^2}-1/2.sqrt{(2/3)^2}`
`=2/5. 5/4-1/2. 2/3`
`=1/2-1/3`
`=1/6`
---------------------------------
`\qquad 0,5\sqrt{0,04}+5\sqrt{0,36}`
`=0,5.0,2+5.0,6`
`=0,1+3`
`=3,1`
---------------------------------
`\qquad -4sqrt{-25/-16}+5sqrt{-9/-25}`
`=-4sqrt{25/16}+5sqrt{9/25}`
`=-4sqrt{(5/4)^2}+5sqrt{(3/5)^2}`
`=-4. 5/4+5. 3/5`
`=-5+3`
`=-2`
