已知:x2+y2=7,xy=-2.求7x2-3xy-2y2-11xy-5x2+4y2的值.

8个回答

  • 解题思路:(1)可将x2+y2=7,xy=-2看做一个方程组求出x,y的值,再代入7x2-3xy-2y2-11xy-5x2+4y2求值;

    (2)将(x2+y2)和xy分别看做一个整体求解.

    ∵x2+y2=7,xy=-2,

    ∴7x2-3xy-2y2-11xy-5x2+4y2

    =(7x2-5x2)+(4y2-2y2)-(3xy+11xy),

    =2x2+2y2-14xy,

    =2(x2+y2)-14xy,

    =2×7-14×(-2),

    =14+28=42.

    点评:

    本题考点: 整式的混合运算—化简求值.

    考点点评: 解答此题需要先化简,即先合并同类项,然后应用整体思想求值.