（a-3b)^2+(3a+b)^2-(a+5b)^2+(a-5b)^2
=a^2-6ab+9b^2+9a^2+6ab+b^2+[(a-5b)^2-(a+5b)^2]
=10a^2+10b^2+[(a-5b+a+5b)(a-5b-a-5b)]
=10a^2+10b^2-20ab
=10(a-b)^2