Location: f(k)f(l) sign change (For continuous quadratic with two real roots, f(k)f(l)<0 ⇔ one root lies)

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For continuous quadratic with two real roots, f(k)f(l)<0 ⇔ one root lies strictly between k and l (when f(k),f(l) nonzero).

What each symbol means

SymbolWhat it stands for
fQuadratic
k,lReal test points

When to use this

Two distinct real roots; f(k),f(l) ≠ 0 typically.