Bài 3: Chọn câu đúng.
A. (5x – 4)2 – 49×2 = -8(3x + 1)(x + 2)
B. (5x – 4)2 – 49×2 = (3x – 1)(x + 2)
C. (5x – 4)2 – 49×2 = -8(3x – 1)(x – 2)
D. (5x – 4)2 – 49×2 = -8(3x – 1)(x + 2)
Bài 3: Chọn câu đúng. A. (5x – 4)2 – 49×2 = -8(3x + 1)(x + 2) B. (5x – 4)2 – 49×2 = (3x – 1)(x + 2) C. (5x – 4)2 – 49×2 = -8(3x – 1)(x – 2) D. (5x –
By Maya
Ta có (5x – 4)2 – 49x2 = (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)
VẬY NÊN TA CHỌN ĐÁP ÁN : D
Đáp án:
Bài 3: Chọn đáp án 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)`
Đáp án : D
Giải thích các bước giải:
Ta có:
`(5x – 4)² – 49x² = (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)`
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