a:
\(a^3+a^2c-abc+b^2c+b^3\)
\(=\left(a+b\right)\left(a^2-ab+b^2\right)+c\left(a^2-ab+b^2\right)\)
\(=\left(a^2-ab+b^2\right)\left(a+b+c\right)=0\)(vì a+b=c=0)
câu b bn xem ở link này nha!
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\(a^3+a^2c-abc+b^2c+b^3\)
\(=\left(a^3+b^3\right)\left(a^2c-abc+b^2c\right)\)
\(=\left(a+b\right)\left(a^2-ab+b^2\right)+c\left(a^2-ab+b^2\right)\)
\(\Rightarrow\left(a+b+c\right)\left(a^2-ab+b^2\right)=0\)( vì a+b+c=0)
Vậy \(a^3+a^2c-abc+b^2c+b^3=0\left(đpcm\right)\)
\(b,A=bc\left(a+d\right)\left(b-c\right)-ac\left(b+d\right)\left(a-c\right)+ab\left(c+d\right)\left(a-b\right)\)
\(=bc\left(a+d\right)\left[\left(b-a\right)+\left(a-c\right)\right]-ac\left(a-c\right)\left(b+d\right)+ab\left(c+d\right)\left(a-b\right)\)
\(=-bc\left(a+d\right)\left(a-b\right)+bc\left(a+d\right)\left(a-c\right)-ac\left(a-c\right)\left(b+d\right)+ab\left(c+d\right)\left(a-b\right)\)
\(=b\left(a-b\right)\left[a\left(c+d\right)-c\left(a+d\right)\right]+c\left(a-c\right)\left[b\left(a+d\right)-a\left(b+d\right)\right]\)
\(=b\left(a-b\right)\cdot d\left(a-c\right)+c\left(a-c\right)\cdot d\left(b-a\right)\)
\(=d\left(a-b\right)\left(a-c\right)\left(b-c\right)\)