=61×(42+57)=61×99=61×(100-1)=61×100-61=6100-61=6039 里面运用两次乘法分配律。
等于61×（12+57）=61×99=61×（100-1）=6100-61=6039所以结果等于6039
61×42+57×61=61×（42+57）=61×99=61×（100-1）=61×100-61×1=6100-61=6039
