To prove the CAST Diagram
On your SHARP EL535 calculator press . Then type in .
For X_Start: type in 90 and .
For X_Step: type in 15 and .
Look at the answers between -90 and 0 – are they all positive or all negative?
They are all negative: therefore sin is negative between -90 and 0.
Between 0 and 90 sin is positive,
between 90 and 180 sin is ?
between 180 and 270 sin is ?
and between 270 and 360 sin is?
Press twice then type in . Leave your X_Start and X_Step as they were for sin, so press twice. Again look at the ANS column and say between -90 and 0 whether cos is positive or negative; for 0 to 90, 90 to 180, 180 to 270 and 270 to360.
Repeat the process for tan.
Now write down all your information in a table:
|-90 – 0||–||+||–||cos||4|
|0 – 90||+||+||+||all||1|
|90 – 180||+||–||–||sin||2|
|180 – 270||–||–||+||tan||3|
|270 – 360||–||+||–||cos||4|
Now we can summarise our results in a the form of a cartesian plane, where each quadrant represent the positive function:
And that is where the the CAST diagram comes from. The line between C and A represents 0 or 360, the line between A and S represents 90, the line between S and T represents 180 and the line between T and C represents 270 or -90.