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a) \(\left(A+B\right)^2=\left(A+B\right)\left(A+B\right)=A^2+AB+AB+B^2=A^2+2AB+B^2\)
b) \(\left(A+B\right)^3=\left(A+B\right)^2\left(A+B\right)=\left(A^2+2AB+B^2\right)\left(A+B\right)\)( NHÂN ra nốt hộ mk nha ) :D !
c)\(\left(A+B\right)\left(A-B\right)=A^2+AB-AB-B^2=A^2-B^2\)
ý d tương tự nha :D !
\(a,a^3-7a-6\)
\(\Leftrightarrow a^3+a^2-a^2-a-6a-6\)
\(\Leftrightarrow a^2\left(a+1\right)-a\left(a+1\right)-6\left(a+1\right)\)
\(\Leftrightarrow\left(a+1\right)\left(a^2-a-6\right)\)
\(\left(x+1\right)\left(x+2\right)\left(x-3\right)\)
\(b,a^3+4a^2-7a-10\)
\(\Leftrightarrow a^3+5a^2-a^2-5a-2a-10\)
\(\Leftrightarrow a^2\left(a+5\right)-a\left(a+5\right)-2\left(a+5\right)\)
\(\Leftrightarrow\left(a+5\right)\left(a+1\right)\left(a-2\right)\)
\(d,\left(a^2+a\right)^2+4\left(a^2+a\right)-12\)
Đặt a^2+a=y ta có
y^2+4y-12=(y+6)(y-2)
<=> (a^2+a+6)(a^2+a-2)
<=> (a^2+a+6)(x-1)(x+2)
câu a sử dụng hdt số 3
cau b tach 4=2*2
cau c tach 9=3*3
cau d tach 1/4=1/2*1/2
\(\left(a+b+c\right)\left(ab+bc+ca\right)-abc\)
\(=\left(a+b+c\right)\left(ab+bc\right)+\left(a+b+c\right)ac-abc\)
\(=\left(ab+b^2+bc\right)\left(a+c\right)+\left(a+c\right)ac+abc-abc\)
\(=\left(a+c\right)\left(ab+b^2+bc+ac\right)\)
\(=\left(a+b\right)\left(b+c\right)\left(c+a\right)\)
\(a.=a\left(a^2+a+1\right)\)
\(=a\left(a+\frac{1}{2}\right)^2+\frac{3}{4}\)
\(b.=\left(a+b\right)^2-9\)
\(=\left(a+b\right)^2-3^2\)
\(=\left(a+b-3\right)\left(a+b+3\right)\)
\(c.=a\left(b-1\right)+b\left(b-1\right)\)
\(=\left(a+b\right)\left(b-1\right)\)
\(d.=a^2-3a-4a+12\)
\(=a\left(a-3\right)-4\left(a-3\right)\)
\(=\left(a-4\right)\left(a-3\right)\)