18.原式=3√2-1+3-2√2-2
=(3√2-2√2)+(-1+3-2)
=√2
19.原式=a²-4+2a-a²
=2a-4
当a=√2+2时
原式=2(√2+2)-4
=2√2+4-4
=2√2
18、19怎么算,
18.原式=3√2-1+3-2√2-2
=(3√2-2√2)+(-1+3-2)
=√2
19.原式=a²-4+2a-a²
=2a-4
当a=√2+2时
原式=2(√2+2)-4
=2√2+4-4
=2√2