f(1)=2
f(0.5)=0.625
f'(0.5)=0
Drei Gleichungen, drei Unbekannte.
$$f(x) = ax^4+ bx+ c$$
$$ f'(x)=4ax^3+b $$
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$$f(1)=2\Rightarrow a+b+c=2~~~~~(I)$$
$$f(0.5)=0.625\Rightarrow 0.0625a+0.5b+c=0.625~~~~~(II)$$
$$f'(0.5)=0 \Rightarrow 0.5a+b=0~~~~~(III)$$
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$$ (I)-(II)~~~~~0.9375a+0.5b=1.375~~~~(IV) $$
$$0.5a+b=0~~~~~(III)$$
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$$ 2\cdot(IV) ~~~~~1.875a+b =2.75~~~~~(V)$$
$$ -(III)~~~~~ -0.5a-b=0~~~~~(VI)$$
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$$ (V)+(VI)~~~~~ 1.375a=2.75~~~~~|:1.375 $$
$$ a=2 $$
$$ a=2 \text{ in } (III): ~~~~0.5\cdot2+b=0 \Rightarrow b=-1 $$
$$ a; b \text{ in } (I): ~~~~2-1+c=2\Rightarrow c=1 $$
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$$ f(x)=2x^4-x+1$$