\[\boxed{\mathbf{19}\mathbf{.}}\]
\[\textbf{а)}\ f(x) = x^{2};\ \ x_{0} = 0\]
\[f^{'}(x) = 2x;\]
\[y_{0} = f(0) = 0;\]
\[k = f^{'}(0) = 2 \cdot 0 = 0.\]
\[y - 0 = 0(x - 0)\]
\[y = 0.\]
\[Уравнение\ касательной:\ \ y = 0.\]
\[\textbf{б)}\ f(x) = x^{2};\ \ x_{0} = 1\]
\[f^{'}(x) = 2x;\]
\[y_{0} = f(1) = 1;\]
\[k = f^{'}(1) = 2 \cdot 1 = 2.\]
\[y - 1 = 2(x - 1)\]
\[y - 1 = 2x - 2\]
\[y = 2x - 1.\]
\[Уравнение\ касательной:\ \ \]
\[y = 2x - 1.\]
\[\textbf{в)}\ f(x) = x^{2};\ \ x_{0} = 2\]
\[f^{'}(x) = 2x;\]
\[y_{0} = f(2) = 4;\]
\[k = f^{'}(2) = 2 \cdot 2 = 4.\]
\[y - 4 = 4(x - 2)\]
\[y - 4 = 4x - 8\]
\[y = 4x - 4.\]
\[Уравнение\ касательной:\ \]
\[\ y = 4x - 4.\]
\[\textbf{г)}\ f(x) = x^{2};\ \ x_{0} = - 1\]
\[f^{'}(x) = 2x;\]
\[y_{0} = f( - 1) = 1;\]
\[k = f^{'}( - 1) = - 2.\]
\[y - 1 = - 2(x + 1)\]
\[y - 1 = - 2x - 2\]
\[y = - 2x - 1.\]
\[Уравнение\ касательной:\]
\[\ \ y = - 2x - 1.\]