`a)sqrt{9+4sqrt2}=sqrt{8+2.2sqrt2+1}=sqrt{(2sqrt2+1)^2}=2sqrt2+1` $\\$ `b)sqrt{3+sqrt8}=sqrt{(6+2sqrt8)/2}=sqrt{(4+2.2.sqrt2+2)/2}=sqrt{(2+sqrt2)^2/2}=(2+sqrt2)/sqrt2=sqrt2+1` $\\$ `c)sqrt{8-sqrt{28}}=sqrt{8-2sqrt7}=sqrt{7-2sqrt7+1}=sqrt{(sqrt7-1)^2}=sqrt7-1`