Equation of normal (non-vertical tangent) (y - y0 = (-1/f'(x0))(x - x0), f'(x0) ≠ 0)

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y - y0 = (-1/f'(x0))(x - x0), f'(x0) ≠ 0

What each symbol means

SymbolWhat it stands for
y_0f(x_0)
f'(x_0)Tangent slope (non-zero)

When to use this

f'(x0) finite and non-zero. If f'(x0)=0, normal is vertical x=x0. If tangent vertical, normal is horizontal.