18.计算：√12-√18-√0.5+√1/3
=2√3-3√2-√2/2+√3/3;
=7√3/3-7√2/2;
19.解方程：（x+2）（2x-1）=3
2x²-x+4x-2=3;
2x²+3x-5=0;
(2x+5)(x-1)=0;
x1=-5/2;x2=1;