a=√2=4/(2√2)=4/(√2+√2)
b=√7-√3=4/(√7+√3)
c=√6-√2=4/(√6+√2)
由 0<(√2+√2)<(√6+√2)<(√7+√3)
得 4/(√2+√2)>4/(√6+√2)>4/(√7+√3)
即 a>c>b