3x²-10xy-5y²=3[x²-10xy/3+(5y/3)²]-3×25y²/9-5y²=3(x-5y/3)²-40y²/3=3[(x-5y/3)²-(2√10y/3)²]=3(x-5y/3+2√10y/3)(x-5y/3-2√10y/3)