- Double-angle formulae. These are the ones you need here:
- When the various trig functions are positive. Here's the diagram:
(Remember ACTS starting in QI and going clockwise.)
So for #3, A is in QIII, so only tangent is positive: sine and cosine are both negative. So if (Pythagoras) and .
Using :For # 4, x is in QIV, so only cosine is positive; sine and tangent are negative. So (Pythagoras)
(You can check this out.)For #5, all functions are positive in QI. So (Pythagoras)
(Check my working)Grandad