Lời giải.
`1.`
`3\sqrt{5a}-5\sqrt{45a}+4\sqrt{20a}(a≥0)`
`=3\sqrt{5a}-5\sqrt{9.5a}+4\sqrt{4.5a}`
`=3\sqrt{5a}-5.\sqrt{9}.\sqrt{5a}+4.\sqrt{4}.\sqrt{5a}`
`=3\sqrt{5a}-5.3.\sqrt{5a}+4.2.\sqrt{5a}`
`=3\sqrt{5a}-15.\sqrt{5a}+8.\sqrt{5a}`
`=\sqrt{5a}.(3-15+8)`
`=-4\sqrt{5a}`
`=-4\sqrt{5}\sqrt{a}`
`2.`
`2\sqrt{12}+3\sqrt{32}-\sqrt{18}+\sqrt{50}`
`=2\sqrt{4.3}+3\sqrt{16.2}-\sqrt{9.2}+\sqrt{25.2}`
`=2\sqrt{4}.\sqrt{3}+3\sqrt{16}.\sqrt{2}-\sqrt{9}.\sqrt{2}+\sqrt{25}.\sqrt{2}`
`=2.2.\sqrt{3}+3.4.\sqrt{2}-3.\sqrt{2}+5.\sqrt{2}`
`=4\sqrt{3}+12\sqrt{2}-3\sqrt{2}+5\sqrt{2}`
`=4\sqrt{3}+14\sqrt{2}.`
