Solve the problem.
-$\mathrm { A } = \pi \mathrm { r } ^ { 2 }$, where $\mathrm { r }$ is the radius, in centimeters. By approximately how much does the area of a circle decrease when the radius is decreased from $4.0 \mathrm {~cm}$ to $3.9 \mathrm {~cm}$ ? (Use $3.14$ for $\pi$.)
A) $1.3 \mathrm {~cm} ^ { 2 }$
B) $2.3 \mathrm {~cm} ^ { 2 }$
C) $2.5 \mathrm {~cm} ^ { 2 }$
D) $2.7 \mathrm {~cm} ^ { 2 }$

Question

Quizplus · Accepted Answer

The area of the circle with radius 4.0 cm is $A_1 = \pi (4.0)^2 = 16 \pi \mathrm {~cm} ^ { 2 }$. The area of the circle with radius 3.9 cm is $A_2 = \pi (3.9)^2 = 15.21 \pi \mathrm {~cm} ^ { 2 }$. The decrease in area is $A_1 - A_2 = 16 \pi - 15.21 \pi = 0.79 \pi \mathrm {~cm} ^ { 2 } \approx 2.5 \mathrm {~cm} ^ { 2 }$.

Solve the Problem A=πr2\mathrm { A } = \pi \mathrm { r } ^ { 2 }A=πr2

Solve the Problem $\mathrm { A } = \pi \mathrm { r } ^ { 2 }$