関連ワード:  first 5 triangular numbers    1st 5 triangular numbers    first five triangular numbers mean    write first five triangular numbers    list the first 5 triangular numbers    sum of first 5 triangular numbers    mean of first 5 triangular numbers    the first five triangular numbers are 1 3 6 10 and    demonstrate first five triangular numbers using dots    represent first five triangular numbers pictorially