＾2+c＾2－a＾2=b＾2+c＾2－(b+c)＾2=-2bc ,那么c＾2+a＾2-b＾2=-2ac
a＾2+b＾2-c＾2=-2ab
1/（b＾2+c＾2－a＾2）+1/(c＾2+a＾2-b＾2）+1/(a＾2+b＾2-c＾2)
=1/(-2bc)+1/(-2ac)+1/(-2ab)
=(a+b+c)/(-2abc)=0
2.假设n为正整数,求证1/(1*3)+1/(3*5)+……+1/((2n+1)(2n-1))< 1/2
1/(1*3)+1/(3*5)+……+1/((2n+1)(2n-1))
={(1-1/3)+(1/3-1/5)+.(1/(2n-1)-1/(2n+1))}/2
={1-1/(2n+1)}/2 < 1/2 OK了,给分.