In what way has the spread of coal-powered factories been characteristic of the modern era

Modern era or period refers to a time after post-classical history. It can be divided in the early era (16 Th. century) and late modern era (18 Th. century).

Some examples of historical facts in the late modern era are American, French and Russian Revolution and the Industrial Revolution.

The spread of coal-powered factories has been characteristic of the modern era because coal was used to heat buildings, generate electricity and for steam engines. It generates energy for industry and transportation.  

254.34

step-by-step explanation:

the formula for area of a circle is given by

a = pi * r^2

a = pi * 9^2

a = 81*pi

using 3.14 as an approximation for pi

a = 3.14*81

a = 254.34