已知2x²－4xy=20,6xy+8y²=-19,代数式3x²+8y²=_______怎么做

已知2x²－4xy=20,6xy+8y²=-19,代数式3x²+8y²=_______怎么做
已知2x²－4xy=20,6xy+8y²=-19,代数式3x²+8y²=_______怎么做

已知2x²－4xy=20,6xy+8y²=-19,代数式3x²+8y²=_______怎么做
3x²+8y²=3x²-6xy+6xy+8y²=3/2(2x²-4xy)+(6xy+8y²)=3/2×20+(-19)=11如果满意记得采纳!