[boxed{a+b}]

$a+b$,

[a+b]

begin{equation}
    a+b
end{equation}

begin{equation*}
    a+b
end{equation*}

begin{eqnarray}
    a+b \
    c+d
end{eqnarray}

begin{align}
    a+b \
    c+d
end{align}

begin{equation}
    left(
    begin{matrix}
        a &b \
        c &d
    end{matrix}
    right)
end{equation}

$boxed{a+b}$

[boxed{a+b}]

begin{equation}
    boxed{a+b}
end{equation}

begin{equation*}
    boxed{a+b}
end{equation*}

begin{eqnarray}
    boxed{a+b} \
    boxed{c+d}
end{eqnarray}

begin{align}
    boxed{a+b} \
    boxed{c+d}
end{align}

begin{equation}
boxed{
    left(
    begin{matrix}
        a &b \
        c &d
    end{matrix}
    right)
}
end{equation}