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\(P\left(x\right)=\left(4x+1\right)\left(12x-1\right)\left(3x+2\right)\left(x+1\right)-4\)
\(=\left[\left(4x+1\right)\left(3x+2\right)\right].\left[\left(12x-1\right)\left(x+1\right)\right]-4\)
\(=\left(12x^2+8x+3x+2\right).\left(12x^2+12x-x-1\right)-4\)
\(=\left(12x^2+11x+2\right).\left(12x^2+11x-1\right)-4\)
Đặt \(12x^2+11x=t\), ta có:
\(\left(t+2\right)\left(t-1\right)-4\)
\(=t^2-t+2t-2-4=t^2+t-6\)
\(=t^2-2t+3t-6\)
\(=t\left(t-2\right)+3\left(t-2\right)=\left(t-2\right)\left(t+3\right)\)
Thay \(t=12x^2+11x\), ta được:
\(P\left(x\right)=\left(12x^2+11x-2\right)\left(12x^2+11x+3\right)\)
Đs...
=(4x+1)(3x+2)(12x-1)(x+1)-4
=(12x2+11x+2)(12x2+11x-1)-4
đặt a=12x2+11x+2
khi đó đa thức trở thành:
a(a-3)-4
=a2-3a-4
=a2+a-4a-4
=a(a+1)-4(a+1)
=(a+1)(a-4)
thay x vào là ok
a) \(x^2-8y^2+6x+9\)
\(=\left(x^2+6x+9\right)-8y^2\)
\(=\left(x+3\right)^2-\left(\sqrt{8}\cdot y\right)^2\)
\(=\left(x+3+\sqrt{8}y\right)\left(x+3-\sqrt{8}y\right)\)
Đặt \(A=\left(x+2\right)\left(x+3\right)\left(x+4\right)\left(x+5\right)-24\)
\(A=\left(x^2+7x+10\right)\left(x^2+7x+12\right)-24\)
Đặt \(x^2+7x+10=y\)
\(\Rightarrow\)\(A=y.\left(y+2\right)-24\)
\(A=y^2+2y+1-25\)
\(A=\left(y+1\right)^2-5^2\)
\(A=\left(y+1-5\right)\left(y+1+5\right)\)
\(A=\left(y-4\right)\left(y+6\right)\)
\(\Rightarrow A=\left(x^2+7x+6\right)\left(x^2+7x+16\right)\)
\(A=\left[\left(x^2+x\right)+\left(6x+6\right)\right].\left(x^2+7x+16\right)\)
\(A=\left[x.\left(x+1\right)+6.\left(x+1\right)\right].\left(x^2+7x+16\right)\)
\(A=\left(x+1\right).\left(x+6\right).\left(x^2+7x+16\right)\)
Đặt \(B=\left(4x+1\right)\left(12x-1\right)\left(3x+2\right)\left(x+1\right)-4\)
\(B=\left(12x^2+11x+2\right)\left(12x^2+11x-1\right)-4\)
Đặt \(12x^2+11x-1=a\)
\(\Rightarrow B=a.\left(a+3\right)-4\)
\(B=a^2+3a-4\)
\(B=\left(a^2-a\right)+\left(4a-4\right)\)
\(B=a.\left(a-1\right)+4.\left(a-1\right)\)
\(B=\left(a-1\right)\left(a+4\right)\)
\(\Rightarrow B=\left(12x^2+11x-2\right)\left(12x^2+11x+3\right)\)
Ta có (6x+5)2(3x+2)(x+1)-35
= (36x2+60x+25)(3x2+5x+2)-35 (1)
Đặt a=3x2+5x+2
=> 12a+1= 12(3x2+5x+2)+1 =36x2+60x+25
Thay a=3x2+5x+2 vào (1) ta được
(12a+1).a-35=12a2+a-35
= 12a2-20a+21a-35
= 4a(3a-5)+7(3a-5)
= (3a-5)(4a+7) (2)
Thay 3x2+5x+2=a vào (2) ta được
(9x2+15x+6-5)(12x2+20x+8+7)
= (9x2+15x+1)(12x2+20x+15)
Ta có: \(\left(6x+5\right)^2\left(3x+2\right)\left(x+1\right)-35\)
\(=\left(36x^2+60x+25\right)\left(3x^2+5x+2\right)-35\)(1)
Đặt \(3x^2+5x+2=y\)
\(\left(1\right)=\left(12y+1\right)y-35\)
\(=12y^2+y-35\)
\(=\left(3y-5\right)\left(4y+7\right)\)
\(=\left(9x^2+15x+1\right)\left(12x^2+20x+15\right)\)
\(\left(3x-2\right)\left(4x-3\right)-\left(2-3x\right)\left(x-1\right)-2\left(3x-2\right)\left(x+1\right)\)
\(=\)\(\left(3x-2\right)\left(4x-3\right)+\left(3x-2\right)\left(x-1\right)-\left(3x-2\right)\left(2x+2\right)\)
\(=\)\(\left(3x-2\right)\left(4x-3+x-1-2x-2\right)\)
\(=\)\(\left(3x-2\right)\left(3x-6\right)\)
\(=\)\(3\left(x-2\right)\left(3x-2\right)\)
Chúc bạn học tốt ~
Đặt: \(x^2-6x+1=a;x^2+1=b\)
Khi đó đa thức này có dạng:
\(2a^2+5ab+2b^2=2a^2+4ab+ab+2b^2\)
\(=2a\left(a+2b\right)+b\left(a+2b\right)=\left(a+2b\right)\left(2a+b\right)\)
Thay lại a và b thì được:
\(\left(a+2b\right)\left(2a+b\right)=\left(x^2-6x+1+2x^2+2\right)\left(2x^2-12x+2+x^2+1\right)\)
\(=\left(3x^2-6x+3\right)\left(3x^2-12x+3\right)\)
\(=9\left(x-1\right)^2\left(x^2-4x+1\right)\)
Vậy ...