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\(x^5-4x^3-5x\)
\(=x\left(x^4-4x^2-5\right)\)
\(=x\left(x^4-5x^2+x^2-5\right)\)
\(=x\left[x^2\left(x^2-5\right)+\left(x^2-5\right)\right]\)
\(=x\left(x^2+1\right)\left(x+\sqrt{5}\right)\left(x-\sqrt{5}\right)\)
a/
\(a^4+b^4+c^4-2a^2b^2-2b^2c^2-2c^2a^2.\)
=>\(a^4+b^4+c^4-2\left(ab\right)^2-2\left(bc\right)^2-2\left(ac\right)^2\)
=>\(a^4+b^4+c^4-2\left(ab\right)^2-2\left(bc\right)^2+2\left(ac\right)^2-4\left(ca\right)^2\)
áp dụng hằng đẳng thức \(a^2-b^2-c^2=a^4+b^4+c^4-2\left(ab\right)^2-2\left(bc\right)^2+2\left(ac\right)^2\) ta đc
\(\left(a^2-b^2+c^2\right)-4\left(ac\right)^2\)
=> \(\left(a^2-b^2+c^2-2ac\right)\left(a^2-b^2+c^2+2ac\right)\)
Mấy câu dễ mình làm trước nhé. Mấy câu khó hơn mình trình bày sau :)
1) 2x2 - 5xy - 3y2 = 2x2 + xy - 6xy - 3y2 = x( 2x + y ) - 3y( 2x + y ) = ( 2x + y )( x - 3y )
2) 7x2 + 3xy - 10y2 = 7x2 - 7xy + 10xy - 10y2 = 7x( x - y ) + 10y( x - y ) = ( x - y )( 7x + 10y )
3) x2 + 5x - 2 = ( x2 + 5x + 25/4 ) - 33/4 = ( x + 5/2 )2 - \(\left(\frac{\sqrt{33}}{2}\right)^2\)= \(\left(x+\frac{5}{2}-\frac{\sqrt{33}}{2}\right)\left(x+\frac{5}{2}+\frac{\sqrt{33}}{2}\right)\)
6) x4 + 324 = ( x4 + 36x2 + 324 ) - 36x2 = ( x2 + 18 )2 - ( 6x )2 = ( x2 - 6x + 18 )( x2 + 6x + 18 )
4) x8 + x7 + 1
= x8 + x7 + x6 - x6 + 1
= x6( x2 + x + 1 ) - ( x6 - 1 )
= x6( x2 + x + 1 ) - ( x3 - 1 )( x3 + 1 )
= x6( x2 + x + 1 ) - ( x - 1 )( x2 + x + 1 )( x3 + 1 )
= ( x2 + x + 1 )( x6 - ( x - 1 )( x3 + 1 ) ]
= ( x2 + x + 1 )( x6 - x4 + x3 - x + 1 )
5) x7 + x5 + 1
= x7 + x6 - x6 + x5 + 1
= x5( x2 + x + 1 ) - ( x6 - 1 )
= x5( x2 + x + 1 ) - ( x3 - 1 )( x3 + 1 )
= x5( x2 + x + 1 ) - ( x - 1 )( x2 + x + 1 )( x3 + 1 )
= ( x2 + x + 1 )[ x5 - ( x - 1 )( x3 + 1 ) ]
= ( x2 + x + 1 )( x5 - x4 + x3 - x + 1 )
7) x5 - 5x3 + 4x
= x5 - x3 - 4x3 + 4x
= x3( x2 - 1 ) - 4x( x2 - 1 )
= ( x2 - 1 )( x3 - 4x )
= ( x - 1 )( x + 1 )x( x2 - 4 )
= x( x - 1 )( x + 1 )( x - 2 )( x + 2 )
8) Xin hàng :)
x^2 - 7xy + 10y^2
= (x^2 - 2xy) - (5xy - 10y^2)
= x(x - 2y) - 5y( x - 2y)
= (x - 5y)(x - 2y)
b)4x^2y^2-(x^2+y^2)^2
=(2xy)2-(x2+y2)2
=[2xy-(x2+y2)][2xy+(x2+y2)]
=(2xy-x2-y2)(2xy+x2+y2)
=-(-2xy+x2+y2)(x+y)2
=-(x-y)2(x+y)2
c) x^2+2x-3
=x2-x+3x-3
=x(x-1)+3(x-1)
=(x-1)(x+3)
d) 2a^3-54b^3
=2(a3-27b3)
=2[a3-(3b)3]
=2(a-3b)(a2+3ab+9b2)
a/ x3 + x2 z + y2 z - xyz + y3
= (x + y)(x2 - xy + y2) + z(x2 - xy + y2)
= (x2 - xy + y2)(x + y + z)
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câu nào mình biết mình trl trc nha:
\(\left(x^2+1\right)^2-4x^2\)
\(=x^4+2x^2+1-4x^2\)
\(=x^4-2x^2+1\)
\(\left(x^2-1\right)^2\)
cái này dễ nhưng làm lâu lắm