Đề kiểm tra 15 phút - Đề số 1 - Bài 7 - Chương 2 - Đại số 8
Đề bài
Rút gọn:
1. A=a−ba2+ab.b2+ab(a−b)2A=a−ba2+ab.b2+ab(a−b)2
2. B=(x2+y2x2−y2−1).x−y2y.B=(x2+y2x2−y2−1).x−y2y.
3. C=a2+2ab+b2a2+ab+b2.a3−b37a+7bC=a2+2ab+b2a2+ab+b2.a3−b37a+7b
4. D=(a+12a−2−12a2−2).2a+2a+2.D=(a+12a−2−12a2−2).2a+2a+2.
Hướng dẫn giải
1.
A=a−ba2+ab.b2+ab(a−b)2=(a−b)b(a+b)a(a+b)(a−b)2A=a−ba2+ab.b2+ab(a−b)2=(a−b)b(a+b)a(a+b)(a−b)2=ba(a−b).=ba(a−b).
2.
B=[x2+y2−(x2−y2)x2−y2].x−y2y=2y2(x−y)(x2−y2)2yB=[x2+y2−(x2−y2)x2−y2].x−y2y=2y2(x−y)(x2−y2)2y=yx+y.=yx+y.
3.
C=(a+b)2(a−b)(a2+ab+b2)7(a2+ab+b2)(a+b)=a2−b27.C=(a+b)2(a−b)(a2+ab+b2)7(a2+ab+b2)(a+b)=a2−b27.
4.Ta có: a+12a−2−12a2−2=(a+1)2−12(a2−1)=a2+2a+1−12(a2−1)a+12a−2−12a2−2=(a+1)2−12(a2−1)=a2+2a+1−12(a2−1)=a(a+2)2(a2−1)=a(a+2)2(a2−1)
Vậy D=a(a+2)2(a2−1).2(a+1)a+2=aa−1.D=a(a+2)2(a2−1).2(a+1)a+2=aa−1.