`y' = x^2-4x+1`
a) `d////d' => k=-3`
`=> f'(x_0) = -3 <=> x_0^2-4x_0+1=-3 <=> x_0=2 `
`=> y_0=-22/3`
`=>` PTTT: `y=-3(x-2)-22/3 = -3x-4/3`
b) `x-2y+1 =0 <=> y=1/2 x+1/2`
`k_(d')=1/2 => k_d=y'=f'(x_0) =-1 : 1/2 = -2`
`=> x_0^2-4x_0+1=-2 <=> x_0=1 \vee x_0=3`
TH1: `x_0=1 => y_0 = -14/3`
`=>` PTTT: `y=-2(x-1)-14/3=-2x-8/3`
TH2: `x_0=3=>y_0=-10`
`=>` PTTT: `y=-2(x-3)-10=-2x-4`