Working with PTC Mathcad Prime > Worksheets and Regions > Selecting One or More Regions
  
Selecting One or More Regions
Hold down Ctrl and click a region or click and drag the pointer across a region. A rectangle appears around the selected region. To select multiple regions, use one of the following methods:
Click and drag the pointer across a group of regions.
Hold down Ctrl and click each region that you want to select.
Click the first region, hold down Shift, and then click the last region for a selection. The two regions and everything between is selected.
Press Ctrl+A to select all the regions in the worksheet.