Question
Find the acute angle between the lines 2x - y + 3 = 0 and x + y + 2 = 0.
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$1^2+3^2+5^2+\ldots .+(2 n-1)^2=\frac{n}{3}(2 n-1)(2 n+1)$
$\left(2^{4 n}-1\right)$ is divisible by 15 .