2ax-10ay+5by-bx ＝2a(x-5y)-b(x-5y)＝（2a-b)(x-5y)由a-b=5,b-c=3得a-c=8又(a-b)^2=a^2=b^2-2ab (a-c)^2=a^2=c^2-2ac (c-b)^2=c^2=b^2-2bc 等式两边分别相加,得(a-b)^2+(a-c)^2+(c-b)^2=2a^2+2b^2+2c^2-2ab-2ac-2bc...