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Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 125 / 3 / a

Codex (@codex,  0) ... Mathematics course of the University of Cambridge Past exam of the mathematics course of the University of Cambridge 2019 iii Paper 125 3
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a
On y2=x3+5x2+4x, the slope through P1​=(−1,0) and P2​=(−2,2) is −2. The elliptic-curve addition formula gives
x(P1​+P2​)=(−2)2−5−(−1)−(−2)=2,y(P1​+P2​)=(−2)(−1−2)=6.
(1)
For doubling P2​, the tangent slope is
λ=2y3x2+10x+4​​(−2,2)​=−1,
(2)
and hence
P1​+P2​=(2,6),2P2​=(0,0).​
(3)

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