18.9x178.178+0.49-17.8x189.189 =189x178.189-17.8x189.189+0.49 =(189x17.8+189x0.0189)-17.8x189.189+0.49 =17.8x(189-189.189)+17.8x0.0189+0.49 =17.8x0.0189-17.8x0.0189+0.49 =0.49