Toán Lớp 8: Bài 3: Chọn câu đúng.
A. (5x – 4)^2 – 49x^2 = -8(3x + 1)(x + 2)
B. (5x – 4)^2 – 49x^2 = (3x – 1)(x + 2)
C. (5x – 4)^2 – 49x^2 = -8(3x – 1)(x – 2)
D. (5x – 4)^2 – 49x^2 = -8(3x – 1)(x + 2)
Bài 4: Chọn câu đúng.
A. (3x – 2y)^2 – (2x – 3y)^2 = 5(x – y)(x + y)
B. (3x – 2y)^2 – (2x – 3y)^2 = (5x – y)(x – 5y)
C. (3x – 2y)^2 – (2x – 3y)^2 = (x – y)(x + y)
D. (3x – 2y)^2 – (2x – 3y)^2 = 5(x – y)(x – 5y)
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Giải đáp+Lời giải và giải thích chi tiết:
Bài 3:
(5x-4)^2-49x^2
=(5x-4)^2-(7x)^2
=(5x-4-7x)(5x-4+7x)
=(-2x-4)(12x-4)
=-2(x+2).4(3x-1)
=-8(3x-1)(x+2)
-> Chọn D
Bài 4:
(3x-2y)^2-(2x-3y)^2
=[(3x-2y)-(2x-3y)][(3x-2y)+(2x-3y)]
=(3x-2y-2x+3y)(3x-2y+2x-3y)
=(x+y)(5x-5y)
=5(x+y)(x-y)
-> Chọn A
Bài 3:
(5x – 4)^2 – 49x^2 = (5x – 4)^2 – (7x)^2
= (5x – 4 + 7x)(5x – 4 – 7x)
= (12x – 4)(-2x – 4)
= 4.(3x – 1).(-2)(x + 2)
= -8(3x – 1)(x + 2)
=> Ta chọn D
Bài 4:
(3x – 2y)^2 – (2x – 3y)^2
= (3x – 2y + 2x – 3y)(3x – 2y – (2x – 3y)
= (3x – 2y)^2 – (2x – 3y)^2
= 5(x – y)(x + y)
=> Ta chọn A