`4)sqrt{13-2sqrt{30}}(3sqrt{10}+sqrt{27})`
`=sqrt{10-2sqrt{10.3}+3}(3sqrt{10}+3sqrt3)`
`=3sqrt{(sqrt{10}-sqrt3)^2}(sqrt{10}+sqrt3)`
`=3(sqrt{10}-sqrt3)(sqrt{10}+sqrt3)`
`=3(10-3)`
`=3.7=21`
`5)E=sqrt{5-x}+sqrt{x+3}(-3<=x<=5)`
`<=>E^2=5-x+x+3+2\sqrt{(5-x)(x+3)}`
`<=>E^2=8+2\sqrt{(5-x)(x+3)}`
`2\sqrt{(5-x)(x+3)}>=0`
`=>E^2>=8`
`=>E>=2\sqrt2`
Dấu "=" xảy ra khi `[(5-x=0),(x+3=0):}<=>[(x=5),(x=-3):}`
Vậy `min_E=2sqrt2<=>[(x=5),(x=-3):}`