Answer:
[tex] cos\:b = \frac{4}{5}[/tex]
Step-by-step explanation:
It is given that: a and b are complementary.
[tex] \therefore a + b = 90\degree\\\\\therefore a = 90\degree - b\\\\Assuming\: sin \: ratio \: on\: both\: sides\\\\sin\:a = sin(90\degree - b)\\\\\therefore \frac{4}{5} = sin(90\degree - b)...(\because sin\:a = \frac{4}{5})\\\therefore \frac{4}{5} =cos\:b...\{\because sin(90\degree - \theta) = cos\: \theta\} \\\\\huge\purple{\boxed{\therefore cos\:b = \frac{4}{5}}}\\[/tex]
