√18=√(3²×2)=3√2
√50=√(5²×2)=5√2
√1/2=√(2/2²)=(1/2)×√2
√72=√(6²×2)=6√2
所以原式=(1/3)×3√2-5√2+(1/2)×(1/2)×√2+6√2
=√2-5√2+(1/4)×√2+6√2
=(9/4)√2