Vector sum rule

Parallelogram (or Triangle) Law of addition of vectors.
Also see other figures under the “vectors” tag: the scalar product, right-hand rule, or the divergence and curl of vector fields.

Edit and compile if you like:

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% Author: Izaak Neutelings (September 2020)
\documentclass[border=3pt,tikz]{standalone}
\usepackage{amsmath}
\usepackage{tikz}
\usepackage{physics}
\usetikzlibrary{calc}
\tikzset{>=latex} % for LaTeX arrow head
\usepackage{xcolor}
\colorlet{veccol}{green!45!black}
\colorlet{myred}{red!90!black}
\colorlet{myblue}{blue!90!black}
\colorlet{mypurple}{blue!50!red!80!black!80}
\tikzstyle{vector}=[->,very thick,veccol]
\usetikzlibrary{arrows.meta}
\tikzstyle{thin arrow}=[dashed,thin,-{Latex[length=4,width=3]}]
\begin{document}
% TWO VECTORS SUM
\begin{tikzpicture}[line cap=round]
  \coordinate (O) at (0,0);
  \coordinate (A) at ( -3:2.1);
  \coordinate (B) at ( 55:1.4);
  \coordinate (A+B) at ($(A)+(B)$);
  
  \draw[vector,thin arrow,myred!40] (B) -- (A+B) node[midway,above] {$\vb{a}$};
  \draw[vector,thin arrow,myblue!40] (A) -- (A+B) node[midway,below right=-2] {$\vb{b}$};
  
  \draw[vector,myred] (O) -- (A) node[midway,below] {$\vb{a}$};
  \draw[vector,myblue] (O) -- (B) node[midway,above left=-2] {$\vb{b}$};
  \draw[vector,mypurple] (O) -- (A+B) node[above right=-3] {$\vb{a}+\vb{b}$};
\end{tikzpicture}
% VECTORS
\begin{tikzpicture}[line cap=round]
  \coordinate (O) at (0,0);
  \coordinate (A) at ( -5:2.1);
  \coordinate (B) at ( 55:1.4);
  \coordinate (C) at (130:1.2);
 
 
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