曾幾何時 約6年以前 k(k+y²)-12y⁴ =k²+ky²-12y⁴ =(k+y²/2)²-12y⁴-y⁴/4 =(k+y²/2)²-[12y⁴+y⁴/4] =(k+y²/2)²-[12+1/4]y⁴ =(k+y²/2)²-[49/4]y⁴ =(k+y²/2)²-(7y²/2)² by the formula:a²-b²=(a+b)(a-b) →(k+y²/2)²-(7y²/2)² =(k+4y²)(k-3y²) =(x²+xy+5y²)(x²+xy-2y²) 留言