What is the lateral area of a cone?

The lateral area of a cone would be:
One half of circumference of the base multiplied by the slant height.

Basically-

1/2 [( Pi x D) x Slant height)

Where "D" is the diameter of the base of the cone.

(Pi x D) is the Circumference of the base

Slant height - Length of a line from the base to the top of a prism, shape, etc.

(Pi- means 3.14159...(usually abbreviated 3.14) or 22/7)

you can remember first few numbers of pi by seeing how many letters there are in the sentence: May. I have a large container of coffee