999²-1
= (999-1)(999+1)
= 998 * 1000
= 998000
2006²
= (2000+6)²
= 2000² + 2*2000*6 + 6²
= 4000000 + 24000 + 36
= 4024036
用乘法公式计算:（1）999²-1（2006²）
999²-1
= (999-1)(999+1)
= 998 * 1000
= 998000
2006²
= (2000+6)²
= 2000² + 2*2000*6 + 6²
= 4000000 + 24000 + 36
= 4024036