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421.
Triangle inequality
$\begin{aligned} a+b&>c \\ a+c&>b\\b+c&>a \end{aligned}$
422.
Triangle inequality
$\begin{aligned}|a-b| <& c \\ |a-c| <& b \\ |b-c| <& a \end{aligned}$
423.
Midline of a triangle
$q=\dfrac{a}{2}, q||a$
424.
Law of cosinse for side a
$a^2=b^2+c^2-2bc\cos\gamma$
425.
Law of cosinse for side b
$b^2=a^2+c^2-2ac\cos\beta$
426.
Law of cosinse for side c
$c^2 = a^2 + b^2 - 2ab\cos\gamma$
427.
Law of sines
$\dfrac{a}{\sin\alpha}=\dfrac{b}{\sin\beta}=\dfrac{c}{\sin\gamma}=2R$
428.
Circumscribed circle radius
$R=\dfrac{a}{2\sin\alpha}=\dfrac{b}{2\sin\beta}=\dfrac{c}{2\sin\gamma}$
429.
Circumscribed circle radius
$R=\dfrac{bc}{2h_a} = \dfrac{ac}{2h_b} = \dfrac{bc}{2h_c}$
430.
Inscribed circle radius
$r^2=\dfrac{(p-a)(p-b)(p-c)}{p}, \,\,\text{ and }\,\, p=\dfrac{a+b+c}{2}$
431.
Inscribed circle radius
$r=\dfrac{1}{ha}+\dfrac{1}{h_b}+\dfrac{1}{h_c}$
432.
Sine of α/2
$\sin\dfrac{\alpha}{2} = \sqrt{\dfrac{(p-b)(p-c)}{bc}}, \text{ where } p=\dfrac{a+b+c}{2}$
433.
Cosine α/2
$\cos\dfrac{\alpha}{2} = \sqrt{\dfrac{p(p-a)}{bc}}, \text{ where } p=\dfrac{a+b+c}{2}$
434.
Tangent α/2
$\tan \dfrac{\alpha}{2}=\sqrt{\dfrac{(p-b)(p-c)}{p(p-a)}}, \text{ where } p=\dfrac{a+b+c}{2}$
435.
Triangle height ha
$h_a=\dfrac{2}{a}\sqrt{p(p-a)(p-b)(p-c)}$
436.
Triangle height ha
$h_a=b\sin\gamma$
437.
Median of a triangle
$m_a=\dfrac{b^2+c^2}{2}-\dfrac{a^2}{2}$
438.
Tirangle area - verison 1
$A=\dfrac{ah_a}{2}$
439.
Triangle area - version 2
$A = \dfrac{ab\sin\gamma}{2}$
440.
Hereons Formula for triangle area
$A = \sqrt{p(p-a)(p-b)(p-c}, \text{ where } p = \dfrac{a+b+c}{2}$