π = Pi (Used in several operations related to circles and spheres.) φ = Phi/Golden Ratio (Used in fields of science, art, architecture, and more.) τ = Tau (π²) e = Euler's number (Used in higher-level mathematics, like calculus or trigonometry √2 = (Not commonly used in mathematics, but is the distance of one side in an isosceles triangle if the other two sides are measuring 1.) i = Imaginary unit (x² + 1 = 0. This is useful for finding square roots of negative numbers.)
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