Tính nhanh : 66^2 ,.....

`a)` `61^2`

`=(60+1)^2`

`=60^2+2.60.1+1`

`=3600+120+1`

`=3721`

`b)` `199^2`

`=(200-1)^2`

`=200^2-2.200.1+1^2`

`=40000-400+1`

`=39601`

`c)` `69^2-31^2`

`=(69-31)(69+31)`

`=38.100`

`=3800`

`d)` `47.53`

`=(50-3)(50+3)`

`=50^2-3^2`

`=250-9`

`=241`

`e)` `34^2+66^2+68.66`

`=34^2+68.66+66^2`

`=34^2+2.34.66+66^2`

`=(34+66)^2`

`=10000`