1,3,6,10,15,21,28,36,45,55,66,78...
第12个是78.
triangular (triangle) numbers can be calculated by adding up consecutive(连贯的） numbers.
例如：第5个triangle number -> 1+2+3+4+5 = 15
1,3,6,10,15,21,28,36,45,55,66,78...
第12个是78.
triangular (triangle) numbers can be calculated by adding up consecutive(连贯的） numbers.
例如：第5个triangle number -> 1+2+3+4+5 = 15