2\1+(3\1+3\2)+(4\1+4\2+4\3)+(5\1+5\2+5\3+5\4)+.+(40\1+40\2+.40\39]=?怎么算?
问题描述:
2\1+(3\1+3\2)+(4\1+4\2+4\3)+(5\1+5\2+5\3+5\4)+.+(40\1+40\2+.40\39]=?怎么算?
答
3\1=5\2ducfhyheye26262