-1/4(2a-b)²+4(a-1/2b)²=1/4[16(a-1/2b)²-(2a-b)²]=1/4(4a-2b-2a+b)(4a-2b+2a-b)=1/4(2a-b)(6a-3b)=3/4(2a-b)(2a-b)=3/4(2a-b)²
