\(a,A=\dfrac{6^5-3^5.31}{12^5+3^5}=\dfrac{\left(3.2\right)^5-3^5.31}{\left(3.2^2\right)^5+3^5}\)

\(=\dfrac{3^5.2^5-3^5.31}{3^5.2^{10}+3^5}\)\(=\dfrac{3^5\left(2^5-31\right)}{3^5\left(2^{10}+1\right)}\)

\(=\dfrac{1}{1025}\)

\(a,A=\dfrac{6^5-3^5.31}{12^5+3^5}=\dfrac{\left(3.2\right)^5-3^5.31}{\left(3.2^2\right)^5+3^5}\)

\(=\dfrac{3^5.2^5-3^5.31}{3^5.2^{10}+3^5}\)\(=\dfrac{3^5\left(2^5-31\right)}{3^5\left(2^{10}+1\right)}\)

\(=\dfrac{1}{1025}\)

\(D=x^4+4xy+4y^2-z^2+2xt-t^2\)

\(=\left[x^2+2.x.2y+\left(2y\right)^2\right]-\left(z^2-2.z.t+t^2\right)\)

\(=\left(x+2y\right)^2-\left(z-t\right)^2\)

\(=\left(x+2y-z+t\right)\left(x+2y+z-t\right)\)

Với \(x=10;y=40;z=30;t=20\):

\(D=\left(10+2.40-30+20\right)\left(10+2.40+30-20\right)\)

\(=\left(10+80-10\right)\left(10+80+10\right)\)

\(=80.100=8000\)

Vậy \(D=8000\)

5. Winter is the worst time for us.

6. It is the ugliest town I know.

7. This is the widest road in the country.

8. What is the smallest animal in the world?

9. Is the whale or the elephant the biggest animal in the world?

10. I am the worst student in class.

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\(a,A=\left(2x-1\right)\left(4x^2+2x+1\right)-7\left(x^3+1\right)\)

\(=\left(2x\right)^3-1-7x^3-7\)

\(=x^3-8\)

\(b,\) Với \(x=-\dfrac{1}{2}\), ta được:

\(A=\left(-\dfrac{1}{2}\right)^3-8\)

\(=-\dfrac{1}{8}-8=-\dfrac{65}{8}\)