题目

Surfing—the art of riding a wave on a pointed board—is the wildest, fastest natural water sport known to man. In recent years, it has developed into a major sport around the world, from Australia to South Africa. Australians brave(挑战) men—eating sharks to ride the green waves Down Under, Hawaiian experts risk(冒…之险) their lives on huge, thirty foot swells(浪涛) against the wind of Oahu; Californians of all ages go out the year-round. In the winter, surf-riders put on life-suits to ride grave waves so cold that their flesh turns blue. Surfing is no sport for weak persons. Swi已知上的最大值比最小值多1，则a＝ ．