Triangular number

A triangular number is a number that is a sum of numbers 1 to n

$$n_\Delta = 1 + 2 + 3 + ... + (n-2) + (n-1) + n$$ Triangular numbers:

$$1_\Delta = 1 = 1$$

$$2_\Delta = 1 + 2 = 3$$

$$3_\Delta = 1 + 2 + 3 = 6$$

$$4_\Delta = 1 + 2 + 3 + 4 = \mathcal{X}$$

$$5_\Delta = 1 + 2 + 3 + 4 + 5 = 12$$

$$6_\Delta = 1 + 2 + 3 + 4 + 5 + 6 = 18$$

$$7_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 = 22$$

$$8_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 = 2X$$

$$9_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 36$$

$$\mathcal{X}_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + \mathcal{X} = 43$$

$$\mathcal{B}_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + \mathcal{X} + \mathcal{B} = 51$$

$$\mathcal{J}_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + \mathcal{X} + \mathcal{B} + \mathcal{J} = 60$$

$$10_\Delta = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + \mathcal{X} + \mathcal{B} + \mathcal{J} + 10 = 70$$