What is the exact value of $cos 135$ ?

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Solution

$c o s 135^{o} = c o s (45^{o} + 90^{o}) = - c o s 45^{o} = - \frac{1}{\sqrt{2}}$
or
$c o s 135^{o} = c o s (180^{o} - 45^{o}) = - c o s 45^{o} = - \frac{1}{\sqrt{2}}$

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