`c)` `(sqrt28-sqrt12-sqrt7)sqrt7+2sqrt21`
`=(2sqrt7-2sqrt3-sqrt7)sqrt7+2sqrt21`
`=14-2sqrt21-7+2sqrt21`
`=14-7`
`=7`
`d)` `sqrt{17-3sqrt32}+sqrt{17+3sqrt32}`
`=sqrt{9-12sqrt2+8}+sqrt{9+12sqrt2+8}`
`=sqrt{3^2-2*3*2sqrt2+(2sqrt2)^2}+sqrt{3^2+2*3*2sqrt2+(2sqrt2)^2}`
`=sqrt{(3-2sqrt2)^2}+sqrt{(3+2sqrt2)^2}`
`=|3-2sqrt2|+|3+2sqrt2|`
`=3-2sqrt2+3+2sqrt2` `(Vì ` `3-2sqrt2 >0` `;` `3+2sqrt2>0` `)`
`=3+3`
`=6`
