根号8分之1+根号8＋根号18
=根号（2/16）+根号(4*2)+根号(9*2)
=根号2/根号16+根号4*根号2+根号9*根号2
=根号2/4+2根号2+3根号2
=(1/4+2+3)根号2
=21/4*根号2
所以
X=21/4