Recalculate one formula for different values with CALC

Below each step is the screen you should see after pressing.

Next to each step is the screen you should see after pressing.

1. 1

   Step 1

   In the ordinary calculation mode, enter $\pi a^2$. Use $a$ for the radius; do not substitute $3$ or $5$ yet.

   Press:
2. 2

   Step 2

   Open CALC with the keys shown. The formula stays above a prompt for $a=$.

   Press:
   
3. 3

   Step 3

   Enter $3$ and press the equals key. The screen shows the exact result $9\times \pi$ and, on the line below, its decimal value $28.27433388231$. The calculator adds that line by itself, so you can write $9\pi {\mathrm{cm}}^2$ or about $28.27{\mathrm{cm}}^2$ without converting.

   Press:
   
4. 4

   Step 4

   Press the equals key again to return to $a=$. Enter $5$ to replace the previous value and press the equals key. The screen shows $25\times \pi$ with $78.53981633974$ below it: $25\pi {\mathrm{cm}}^2$, or about $78.54{\mathrm{cm}}^2$.

   Press: