\(f\left(\frac{5}{7}\right)=f\left(\frac{1}{\frac{7}{5}}\right)=\frac{1}{\left(\frac{7}{5}\right)^2}.f\left(\frac{7}{5}\right)=\frac{25}{49}.f\left(1+\frac{2}{5}\right)=\frac{25}{49}.\left(f\left(1\right)+f\left(\frac{2}{5}\right)\right)\)

Ta có : \(f\left(\frac{2}{5}\right)=f\left(\frac{1}{5}+\frac{1}{5}\right)=f\left(\frac{1}{5}\right)+f\left(\frac{1}{5}\right)=2.f\left(\frac{1}{5}\right)=2.\frac{1}{5^2}.f\left(5\right)=\frac{2}{25}.f\left(1+1+1+1+1\right)\)

\(=\frac{2}{25}.\left(f\left(1\right)+f\left(1\right)+f\left(1\right)+f\left(1\right)+f\left(1\right)\right)=\frac{2}{25}.5=\frac{2}{5}\)

Vậy \(f\left(\frac{5}{7}\right)=\frac{49}{25}.\left(1+\frac{2}{5}\right)=\frac{25}{49}.\frac{7}{5}=\frac{5}{7}\)

a, f(10x) = k.(10x) = 10.(kx) = 10.f(x)

b, f(x₁ + x₂) = k(x₁ + x₂) = kx₁ + kx₂ = f(x₁) + f(x₂)

c, f(x₁ - x₂) = k(x₁ - x₂) = kx₁ - kx₂ = f(x₁) - f(x₂)

\(f\left(x_1\right)=ax_1\) ; \(f\left(x_2\right)=ax_2\) ; \(f\left(x_1x_2\right)=ax_1x_2\)

Để \(f\left(x_1\right)f\left(x_2\right)=f\left(x_1x_2\right)\)

\(\Leftrightarrow ax_1.ax_2=ax_1x_2\)

\(\Leftrightarrow a^2x_1x_2=ax_1x_2\)

\(\Leftrightarrow a^2=a\)

\(\Leftrightarrow\left[{}\begin{matrix}a=0\left(loại\right)\\a=1\end{matrix}\right.\)

Vậy \(a=1\)

a) theo tính chất  ta có: f(0+0)= f(0)+f(0)

=> f(0)=f(0)+f(0)

=> f(0)-f(0)=f(0)+f(0)-f(0)

=> 0=f(0)

hay f(0)=0

b)  f(0)=f(-x+x)=f(-x)+f(x)

=>0=f(-x)+f(x)

=> f(-x)=0-f(x)=-f(x)

c) \(f\left(x_1-x_2\right)=f\left(x_1+\left(-x_2\right)\right)=f\left(x_1\right)+f\left(-x_2\right)=f\left(x_1\right)-f\left(x_2\right)\)

Câu 1/

\(f\left(13\right)=x^{13}\left(x-14\right)+14x^{12}-...-14x+14\)

\(=-x^{13}+14x^{12}-14x^{11}+...-14x+14\)

\(=x^{12}\left(-x+14\right)-14x^{11}+...-14x+14\)

\(=x^{12}-14x^{11}+...-14x+14=...\)

\(=-x+14=1\)

(Bạn để ý quy luật sau các bước rút gọn lần lượt thì mũ chẵn sẽ biến thành hệ số 1, mũ lẻ thành hệ số -1 nên x sẽ có hệ số -1)

Câu 2:

+) \(f\left(-x\right)=f\left(x\right)\) có: \(f_3\left(x\right);f_4\left(x\right);f_6\left(x\right)\)

+) \(f\left(-x\right)=-f\left(x\right)\) có: \(f_1\left(x\right);f_2\left(x\right);f_5\left(x\right)\)

+) \(f\left(x_1+x_2\right)=f\left(x_1\right)+f\left(x_2\right)\) có: \(f_1\left(x\right);f_2\left(x\right)\)

+) \(f\left(x_1x_2\right)=f\left(x_1\right).f\left(x_2\right)\) có: \(f_1\left(x\right);f_3\left(x\right);f_5\left(x\right);f_6\left(x\right)\)

ta có:

\(f\left(x_1\right)=kx_1;f\left(x_2\right)=kx_2=>f\left(x_1-x_2\right)=k.\left(x_1-x_2\right)=kx_1-kx_2\)

vậy \(f\left(x_1-x_2\right)=f\left(x_1\right)-f\left(x_2\right)\)

tick mk nhé

Bài 1:

nếu x₁<x₂=>2018.x₁-3<2018.x₂

=>f(x₁)<f(x₂)

Bài 2:

nếu x dương=>100x²+2 dương

nếu x âm=>100x²+2 dương vì  x² luôn dương

=>f(x)=f(-x)

Bài 3:

nếu x₁<x₂=>-2019x₁+1<2019x₂+1

=>f(x₁)<f(x₂)