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a)
\(\begin{array}{l}\left( {2x - 5y} \right)\left( {2x + 5y} \right) + {\left( {2x + 5y} \right)^2}\\ = \left( {2x + 5y} \right)\left( {2x - 5y + 2x + 5y} \right)\\ = \left( {2x + 5y} \right).4x\\ = 2x.4x + 5y.4x\\ = 8{x^2} + 20xy\end{array}\)
b)
\(\begin{array}{l}\left( {x + 2y} \right)\left( {{x^2} - 2xy + 4{y^2}} \right) + \left( {2x - y} \right)\left( {4{x^2} + 2xy + {y^2}} \right)\\ = {x^3} + {\left( {2y} \right)^3} + {\left( {2x} \right)^3} - {y^3}\\ = {x^3} + 8{y^3} + 8{x^3} - {y^3}\\ = \left( {{x^3} + 8{x^3}} \right) + \left( {8{y^3} - {y^3}} \right)\\ = 9{x^3} + 7{y^3}\end{array}\)
a)A=x3+x2y+y2x+y3+2x2y+2xy2
=x3+3x2y+3xy2+y3
A=(x+y)3
b)=3x2+2x+(x2+2x+1)-(4x2-25)=12
3x2+2x+x2+2x+1-4x2+25=12
4x+26=12
= >4x=6/13
= >x=6,5
\(\begin{array}{l}\left( {x - 2y} \right)\left( {{x^2} + 2xy + 4{y^2}} \right) + \left( {x + 2y} \right)\left( {{x^2} - 2xy + 4{y^2}} \right)\\ = {x^3} - {\left( {2y} \right)^3} + {x^3} + {\left( {2y} \right)^3}\\ = {x^3} - 8{y^3} + {x^3} + 8{y^3}\\ = 2{x^3}\end{array}\)
ĐK: !x! khác !y!
\(B=\frac{x^2}{\left(x-y\right)^2\left(x+y\right)}-\frac{2xy^2}{\left(x-y\right)^2\left(x+y\right)^2}+\frac{y^2}{\left(x-y\right)\left(x+y\right)^2}\) =>\(MSC=\left(x-y\right)^2\left(x+y\right)^2\)
\(B=\frac{x^2\left(x+y\right)-2xy^2+y^2\left(x-y\right)}{MSC}=\frac{x^3+x^2y-2xy^2+y^2x-y^3}{MSC}=\frac{x^3+x^2y-xy^2-y^3}{MSC}\)
\(B=\frac{x^3+x^2y-xy^2-y^3}{MSC}=\frac{x^2\left(x+y\right)-y^2\left(x+y\right)}{MSC}=\frac{\left(x+y\right)^2\left(x-y\right)}{\left(x-y\right)^2\left(x+y\right)^2}=\frac{1}{x-y}\)