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Algebra · Simultaneous equations

Chapter 1 · 4

The idea

Simultaneous equations — one linear, one quadratic

Why a line can meet a curve twice (so expect two solution PAIRS), why substitution is the only route in, and the bracket-squaring discipline that circle equations demand.

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Algebra · Simultaneous equations

Simultaneous equations — one linear, one quadratic

Why a line can meet a curve twice (so expect two solution PAIRS), why substitution is the only route in, and the bracket-squaring discipline that circle equations demand.

Why it works

Up to two crossing points

A line and a parabola (or circle) can cross twice — so the algebra must be allowed to produce two answers. That's the headline difference from linear pairs: the solution is up to two pairs (x,y)(x, y), one per crossing point.

Substitution is forced

Elimination thrives on matching ax+byax + by shapes, but x2x^2 has no partner to cancel against. Instead, substitute the linear equation into the quadratic one. For y=x2−3x+4y = x^2 - 3x + 4 and y=x+1y = x + 1: both equal yy, so set them equal —

x2−3x+4=x+1  ⟹  (x−1)(x−3)=0x^2 - 3x + 4 = x + 1 \;\Longrightarrow\; (x - 1)(x - 3) = 0

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