Show that arccot(2)+arccot(3)=arccot(1)

acot_triangle.svg

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<svg xmlns="http://www.w3.org/2000/svg" 
    xmlns:xlink="http://www.w3.org/1999/xlink" 
    width="320" height="220" viewBox="-0.2 -0.2 3.2 2.2">
    <title>arccot(2)+arccot(3)=arccot(1)</title>
    <g fill="#777"> 
        <rect width="4" height="3" fill="#ccc"/>
        <rect x="0" y="0" width="1" height="1"/>
        <rect x="1" y="1" width="1" height="1"/>
        <rect x="2" y="0" width="1" height="1"/>
    </g>
    <g style="fill-opacity:0.4; stroke-opacity:0.8; stroke-width:0.01"> 
        <polygon points="3,0 2,0 3,1" style="fill:#f00; stroke:#f00;"/>
        <polygon points="0,0 0,2 2,0" style="fill:#0f0; stroke:#0f0;"/>
        <polygon points="3,2 3,1 0,2" style="fill:#00f; stroke:#00f;"/>
    </g>
</svg>