2（x+y)(x+y)-(x-y)²+（x-y)²=2(x+y)²-[(x-y)²-(x-y)²]=2(x²+2xy+y²)-0=2x²+4xy+2y²當(dāng)x=2,y=1/2時(shí),原式=2×2²+4×2×½+2×（½）²=8+4+½=12....為什么(x-y)²-(x-y)²]中間是-不是+-(x-y)²+（x-y)²=-[(x-y)²-(x-y)²]這里用到的是“添括號(hào)”法則：把帶小括號(hào)的兩項(xiàng)放到括號(hào)前是“-”號(hào)的括號(hào)里，兩個(gè)小括號(hào)前面的符號(hào)要改變。所以中間的符號(hào)由原來(lái)的“+”變成“-”號(hào)。