(1.1)²+(1.2)²+(1.3)²+(1.4)²+(1.5)²=(1.3-0.2)²+(1.3-0.1)²+(1.3)²+(1.3+0.1)²+(1.3+0.2)²=5×(1.3)²+2×(0.2)²+2×(0.1)²=5×1.69+2×0.04+2×0.01=8.45+0.08+0.02=8.55
=n*100+20(1+2+3+4+5...n)+n!
学一分钟速算吧，学会了算题快几倍----广育网
