计算:729^2-271^2
729^2-271^2=(729-271)*(729+271)=458*1000
1、解:(a-2b)^2=9 =》a^2-4ab+4b^2=9............(1)
(2a+b)^2=25 => 4a^2+4ab+b^2=25...........(2)
(1)+(2)=5(a^2+b^2)=34
所以a^2+b^2=34/5=6.8
ab的值我还没有想到
2解:3×10^5km/s×3×10^7s×4=8.4×10^13km
3解:(x^2+px+3)(x^2-5x+7)
=x^4-5x^3+7x^2+px^2+px^3-5px^2+7px+3x^2-15x+21
=x^4+(p-5)x^3+(10-5p)x^2+(7p-15)x+21
不含x^2项,即(10-5p)=0,所以p=2
4解:(N+68)^2=654481,所以N+68=809,即N=741
(N+58)(N+78)= (741+58)(741+78)=654381
5解:因为两位数的十位数字比个位数字小1,设十位数字a,个位数字为b
即b-a=1,原来两位数为(ab),新两位数为(ba),
即两位数为(ab)比新两位数为(ba)多9,
又因为新两位数与原两位数的乘积比原两位数的平方多405,
即(ba)(ab)-(ab)(ab)=405
(ab)[(ba)-(ab)]=405
(ba)-(ab)=9
所以原两位数(ab)为45