需要用到一些公式：a^2-2ab+b^2=(a-b)^2,a^2-b^2=(a+b)(a-b),ab+ac+ad=a(b+c+d)
1) 12.5^2-12.5*5+6.25
=12.5^2-2*12.5*2.5+2.5^2
=(12.5-2.5)^2
=10^2
=100
2) 21.5^2-3*21.5+1.5^2-100
=21.5^2-2*21.5*1.5+1.5^2-100
=(21.5-1.5)^2-100
=20^2-10^2
=(20+10)(20-10)
=30*10
=300
3) 125*0.45-12.5*6.7+1.25*122
=12.5*4.5-12.5*6.7+12.5*12.2
=12.5*(4.5+12.2-6.7)
=12.5*10
=125