`c)5\sqrt{x/y}-4sqrt{y/x}+sqrt{1/(xy)}(x,y>0)`
`=(5x)/sqrt{xy}-(4y)/sqrt{xy}+1/sqrt{xy}`
`=(5x-4y+1)/sqrt{xy}`
`a)((sqrtx+sqrty)/(sqrtx-sqrty)-(sqrtx-sqrty)/(sqrtx+sqrty)):(sqrt{xy})/(x-y)(x,y>0,x ne y)`
`=((sqrtx+sqrty)^2/((sqrtx-sqrty)(sqrtx+sqrty))-(sqrtx-sqrty)^2/((sqrtx-sqrty)(sqrtx+sqrty)))*((sqrtx-sqrty)(sqrtx+sqrty))/sqrt{xy}`
`=(x+2sqrt{xy}+y-x+2sqrt{xy}-y)/((sqrtx-sqrty)(sqrtx+sqrty))*((sqrtx-sqrty)(sqrtx+sqrty))/sqrt{xy}`
`=(4sqrt{xy})/sqrt{xy}=4`
`b)(1+(a+sqrta)/(sqrta+1))(1-(a-sqrta)/(sqrta-1))(a>0,a ne 1)`
`=(1+(sqrta(sqrta+1))/(sqrta+1))(1-(sqrta(sqrta-1))/(sqrta-1))`
`=(1+sqrta)(1-sqrta)`
`=1-a`
