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\(x\left(x+4\right)\left(x+6\right)\left(x+10\right)+128\)
\(=x\left(x+10\right)\left(x+4\right)\left(x+6\right)+128\)
\(=\left(x^2+10x\right)\left(x^2+10x+24\right)+128\)
\(=\left(x^2+10x\right)^2+24\left(x^2+10x\right)+128\)
\(=\left(x^2+10x\right)^2+2.\left(x^2+10x\right).12+12^2-16\)
\(=\left(x^2+10x+12\right)^2-4^2\)
\(=\left(x^2+10x+12-4\right) \left(x^2+10x +12+4\right)\)
\(=\left(x^2+10x-8\right)\left(x^2+10x+16\right)\)
\(=\left(x^2+10x-8\right)\left(x^2+2x+8x+16\right)\)
\(=\left(x^2+10x-8\right)\left[x\left(x+2\right)+8\left(x+2\right)\right]\)
\(=\left(x^2+10x-8\right)\left(x+2\right)\left(x+8\right)\)
A= x(x+4)(x+6)(x+10) +128
=[(x(x+10)] [(x+4)(x+6)] +128
=(x^2+10)(x^2+10+24)+128
Đặt: x^2+10+12=y
Ta có: A=(y+12)(y-12)+128
=(y^2-12^2)+128
=y^2-12^2+128
=y^2-16
=y^2-4^2
=(y-4)(y+4)
Thay vào bt A ta có:A= ( x^2+10x+12-4)(x^2+10x+12+4)
=(x^2+10x+8)(x^2+10x+16)
=(x^2+10x+8)(x+8)(x+2)x
ta có
\(x\left(x+4\right)\left(x+6\right)\left(x+10\right)+128=\left(x^2+10x\right)\left(x^2+10x+24\right)+128\)
\(=\left[\left(x^2+10x+12\right)-12\right]\left[\left(x^2+10x+12\right)+12\right]+128\)
\(=\left(x^2+10x+12\right)^2-12^2+128=\left(x^2+10x+12\right)^2-16\)
\(=\left(x^2+10x+12-4\right)\left(x^2+10x+12+4\right)=\left(x^2+10x+8\right)\left(x^2+10x+16\right)\)
\(=\left(x+2\right)\left(x+8\right)\left(x^2+10x+8\right)\)
x . ( x + 4 ) . ( x + 6 ) . ( x + 10 ) + 128
= ( x2 + 10x ) . ( x2 + 10x + 24 ) + 128
đặt x2 + 10x + 12 = y, đa thức đã cho có dạng :
( y - 12 ) . ( y + 12 ) + 128 = y2 - 16 = ( y - 4 ) . ( y + 4 )
= ( x2 + 10x + 16 ) . ( x2 + 10x + 8 ) = ( x + 2 ) . ( x + 8 ) . ( x2 + 10x + 8 )
x(x+4)(x+6)(x+10)+128
=x.(x+10)(x+4)(x+6)+128
=(x2+10x)(x2+10x+24)+128
=(x2+10x)[(x2+10x)+24]+128
=(x2+10x)2+24.(x2+10x)+128
=(x2+10x)2+8.(x2+10x)+16.(x2+10x)+128
=(x2+10x)(x2+10x+8)+16.(x2+10x+8)
=(x2+10x+8)(x2+10x+16)
=(x2+10x+8)(x2+2x+8x+16)
=(x2+10x+8)[x.(x+2)+8.(x+2)]
=(x2+10x+8)(x+2)(x+8)