【導(dǎo)讀】B.2=x2+9y2[來(lái)源:學(xué)§科§網(wǎng)Z§X§X§K]. 7.(a-b)=-[()(a-b)]=-[()2-()2]=__________.請(qǐng)你按以上規(guī)律寫出第④個(gè)算式;把這個(gè)規(guī)律用含字母的式子表示出來(lái);你認(rèn)為中所寫出的式子一定成立嗎?y的值無(wú)關(guān),不給出y的值,也能求出答案.”你認(rèn)為誰(shuí)的說(shuō)法正確?5.4點(diǎn)撥:原式可化為:1.23452+2×5×5+52=(5+5)2,8.解:原式=[(x-3)(x+3)]==x4-81;=2-18x2+x2-4=-17x2-2.當(dāng)x=2時(shí),原式=-17×22-2=-17×4-2=-70.當(dāng)y=25時(shí),原式=2+8×25=2+135=155.10.C點(diǎn)撥:逆用平方差公式,由x2-y2=20得,(x+y)(x-y)=20,因?yàn)閤+y=-5,答案不唯一.如n(n+2)-(n+1)2=-1;=4x2-y2-8x2+6xy-y2+2y2-6xy=-4x2.