• Cursor to 4 on D
• Slide to 1.128 on C at cursor (1.128 is the gauge point “c” marked on the C scale on some slide rules, and is the “square root of 4/π”)

This uses the formula : Area = π d2/4. When dividing by the constant “square root of 4/p” in the example above, we are multiplying by “square root of π/4”. When we use the B scale for our answer, we are squaring this value and thus obtain the value π/4 and the square of the diameter, completing the formula.

Another method:

• Slide left index of C to 1.128 on D
• Cursor to 4 on D

Another method ... a direct use of the formula: Area = π r2

• Slide center index of B under π on A
• Cursor to radius 2 on C