Let "△" be a new operation rule, such as 2△3=2+3+4=9, 5△4=5+6+7+8=26, ..., according to this rule 1△x=15, then x = how much.
设"△"是新的运算规则，如2△3=2+3+4=9，5△4=5+6+7+8=26，…，按此规则1△x=15，那么x=多少．
<result>5</result>