郭敦顒回答:
①4.7×4.3=(5-0 .3)(4+0 .3)=20+0 .3-0 .09=20 .21
②7.4×3.4=(7+0.4)(3+0 .4)=21+4+0 .16=25 .15
③9.3×10.7=(10-0 .7)(10+0 .7)=10 ²-0 .7 ²=99 .51
⑩1.94×1.95-1.91×1.98
=1.94×1.95+0.06×1.95-1.91×1.98-0.09×1.98-0.06×1.95+0.09×198
=2×1.95-2×1.98-0.117+0.1782=2(1.95-1.98)+0.0612
=-0.06+0.0612=0.0012
2、已知a²=11111111115555555556,求a
共20位,10节,用手工开方得,a=3333333334 (10位数)