求证：①(a+b+c)2+(a+b-c)2－(a-b-c)2－(a-b-c)2＝8ab ②（x+y）4+x4+y4=2(x2+xy+y2)2 ③(x-2y)x3－(y-2x)y3=(x+y)(x-y)3 ④3 n+2+5 n+2―3 n―5 n=24(5 n+3 n-1) ⑤a5n+a n+1=(a3 n－a2 n+1)(a2 n+a n+1) 2.己知：a...