p.16 問3
| (1) | $$ (a+b)(c-d)=ac-ad+bc-bd $$ |
| (2) | $$ (a-b)(c-d)=ac-ad-bc+bd $$ |
| (3) | $$ (x+2)(y+3)=xy+3x+2y+6 $$ |
| (4) | $$ (x-1)(y+4)=xy+4x-y-4 $$ |
| (1) | $$ (a+b)(c-d)=ac-ad+bc-bd $$ |
| (2) | $$ (a-b)(c-d)=ac-ad-bc+bd $$ |
| (3) | $$ (x+2)(y+3)=xy+3x+2y+6 $$ |
| (4) | $$ (x-1)(y+4)=xy+4x-y-4 $$ |