Bài 9.
\(a,x^2-9\\=x^2-3^2\\=(x-3)(x+3)\\b,49-x^2\\=7^2-x^2\\=(7-x)(7+x)\\c,y^2-81\\=y^2-9^2\\=(y-9)(y+9)\)
Bài 10.
\(a,y^2-121\\=y^2-11^2\\=(y-11)(y+11)\\b,y^2-169\\=y^2-13^2\\=(y-13)(y+13)\\c,y^2-256\\=y^2-16^2\\=(y-16)(y+16)\)
Bài 8.
\(a,-x^2-4y^2+4xy\)
\(=-\left(x^2+4y^2-4xy\right)\)
\(=-\left(x^2-4xy+4y^2\right)\)
\(=-\left[x^2-2\cdot x\cdot2y+\left(2y\right)^2\right]\)
\(=-\left(x-2y\right)^2\)
\(b,-x^4+2x^2-1\)
\(=-\left(x^4-2x^2+1\right)\)
\(=-\left[\left(x^2\right)^2-2\cdot x^2\cdot1+1^2\right]\)
\(=-\left(x^2-1\right)^2\)
\(=-\left(x-1\right)^2\left(x+1\right)^2\)
\(c,x^4+4-4x^2\)
\(=x^4-4x^2+4\)
\(=\left(x^2\right)^2-2\cdot x^2\cdot2+2^2\)
\(=\left(x^2-2\right)^2\)
6:
a: \(4x^2+20xy+25y^2\)
\(=\left(2x\right)^2+2\cdot2x\cdot5y+\left(5y\right)^2\)
\(=\left(2x+5y\right)^2\)
b: \(25x^2+10xy+y^2\)
\(=\left(5x\right)^2+2\cdot5x\cdot y+y^2\)
\(=\left(5x+y\right)^2\)
7:
a: \(10x-x^2-25\)
\(=-\left(x^2-10x+25\right)\)
\(=-\left(x^2-2\cdot x\cdot5+5^2\right)\)
\(=-\left(x-5\right)^2\)
b: \(-x^2-2xy-y^2\)
\(=-\left(x^2+2\cdot x\cdot y+y^2\right)\)
\(=-\left(x+y\right)^2\)
c: \(-x^2+20x-100\)
\(=-\left(x^2-20x+100\right)\)
\(=-\left(x^2-2\cdot x\cdot10+10^2\right)\)
\(=-\left(x-10\right)^2\)
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