6x7=8x9-(6+7+8+9) 7x8=9x10-(7+8+9+10)

Answer 1

Left-hand side:
n(n+1)
=n²+n

Right-hand side:
(n+2)(n+3)-(n+n+1+n+2+n+3)
= n²+5n+6 - (4n+6)
= n²+n

Therefore the relation is an identity (i.e. true for all values of n)