New posts in Proof Verification

Where is the mistake in my 'derivation' of $(uv)' = u'v$?

$n^n<(n-1)^{n+1}$ for integer $n\ge5$

Prove that if $S$ and $T$ are subspaces of $V$, then so is $S\cap T$

Show in a group $G$ that $(a^{-1})^{-1} = a $.

Prove $\sum_\text{cyc}\frac{1}{3-ab}\le\frac{3}{2}$

Proof that $X \subseteq Y$ if and only if $X \cup Y = Y$

Is this series for Pi correct? And who has done it before?

Is this a valid proof that A = B given A ∩ B = A ∪ B?

What is wrong with this proof that symmetric matrices commute?

If $\{x_n\}$ is an increasing sequence and $\lim_{n\to\infty}x_n=L$, then $L$ is an upper bound of $\{x_n\}$