｛4（X-Y)^5+(Y-X)^4}/2(X-Y)^2
＝｛4（X-Y)^5+(X-Y)^4}/2(X-Y)^2
＝｛4（X-Y)^3+(X-Y)^2}（X-Y)^2 /2(X-Y)^2
＝｛4（X-Y)^3+(X-Y)^2}/2
＝2（X-Y)^3+ 0.5(X-Y)^2