69 * 69 = 69^2 = 69 * 68 + 69 = 69 + 68 * 68 + 69 = 2*69 + 68*68
-> 2*(69) + 2*(68) + 2*(67) +...+ 2*(2) + 2
= 2*(69+68+67+66+...+2+1)
= 2*(T(69)) T(n) for Triangular number
=> T(69) = (69 * 69)÷2
=> T(n) = n²/2
69 * 69 = 69^2 = 69 * 68 + 69 = 69 + 68 * 68 + 69 = 2*69 + 68*68
-> 2*(69) + 2*(68) + 2*(67) +...+ 2*(2) + 2
= 2*(69+68+67+66+...+2+1)
= 2*(T(69)) T(n) for Triangular number
=> T(69) = (69 * 69)÷2
=> T(n) = n²/2