Đáp án:
e. 0
f. 0
Giải thích các bước giải:
\(\begin{array}{l}
a.A = {\left( {x - 10} \right)^2} - x\left( {x + 80} \right) = {x^2} - 20x + 100 - {x^2} - 80x\\
= 100 - 100x\\
Thay:x = 0,98 = \dfrac{{98}}{{100}}\\
\to A = 100 - 100.\dfrac{{98}}{{100}} = 100 - 98 = 2\\
b.B = {(2x + 9)^2} - x(4x + 31)\\
= 4{x^2} + 36x + 81 - 4{x^2} - 31x\\
= 5x + 81\\
Thay:x = - 16,2 = - \dfrac{{162}}{{10}}\\
\to B = 5.\left( { - \dfrac{{162}}{{10}}} \right) + 81 = - 81 + 81 = 0\\
c.C = 4{x^2} - 28x + 49 = {\left( {2x} \right)^2} - 2.2x.7 + {7^2}\\
= {\left( {2x - 7} \right)^2}\\
Thay:x = 4\\
\to C = {\left( {2.4 - 7} \right)^2} = 1\\
d.D = {x^3} - 9{x^2} + 27x - 27\\
= {x^3} - 3.{x^2}.3 + 3.x{.3^2} - {3^3}\\
= {\left( {x - 3} \right)^3}\\
Thay:x = 5\\
\to D = {\left( {5 - 3} \right)^3} = {2^3} = 8\\
e.E = 8 - 12x + 6{x^2} - {x^3}\\
= {2^3} - {3.2^2}.x + 3.2.{x^2} - {x^3}\\
= {\left( {2 - x} \right)^3}\\
Thay:x = 2\\
\to E = {\left( {2 - 2} \right)^3} = 0\\
f.F = - {x^3} + 3{x^2} - 3x + 1\\
= - \left( {{x^3} - 3{x^2} + 3x - 1} \right)\\
= - {\left( {x - 1} \right)^3}\\
Thay:x = 1\\
\to F = - {\left( {1 - 1} \right)^3} = 0
\end{array}\)
