How do I know whether to use sin, cos or tan in a problem?

You draw the right angled triangle

You mark you angle.  You mark the 3 sides as opp,adj,hyp

you put you side lengths in the right places.

And then you use the ratio (sin , tan cos) that fits with the sides that you need to use.

This is one way to remember the ratios

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Some old hobos, 

Can't always hide

Their old age.

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Sin = o/h

Cos = a/h

Tan = o/a

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