（√18-√27）÷√6+8√1/2 =(3√2-3√3)/√6+4√2=3/√3-3/√2+4√2=√3-3/2*√2+4√2=√3+5/2*√2
（√18-√27）÷√6+8√1/2 =(3√2-3√3）×√6/6+4√2=√3-3/2√2+4√2=√3+5√2/2
