What is the net area between #f(x) = xe^x-3x # and the x-axis over #x in [1, 5 ]#?

Question

Aurora Alvarado · Accepted Answer

The net area between #xe^x - 3x# and the x-axis over #x in [1,5]# is equal to #color(purple)(5(ln5 - 1) - ln(3)(ln(ln3) - 1) + (3ln^2 3 - 75)/2#.

Let #A# be the given net area. Then, #A = int_(x_1)^(x_2) xe^x - 3x dx#. Now, since the area wanted is the area above the x-axis, that means we have find the minimum and maximum values of #x in [1,5]#, #x_1# and #x_2# respectively, such that #f(x_1)# and #f(x_2)# are both bigger than #0#. Let's plug in #x_1 = 1# and #x_2 = 5# firstly; #f(1) = e-3<0# #f(5) = 5e^5 - 15 > 0# Clearly, #x_1# has to be bigger than #1#. Because #xe^x# increases exponentially at a much faster rate then #3x#, it means that #f(x)# is monotonic on the interval #[1, +oo]#. We don't need the exact interval of monotony to be able to find #A#. In order to find the exact value of #x_1#, we must solve the equation #alphae^alpha - 3alpha = 0# : #alpha(e^alpha - 3) = 0# The two possible solutions are #0# and #ln 3#. However, #x_1 in [1,5] => x_1 = ln 3#. #A# has now become #A = int_ln 3^5 xe^x - 3x dx#. Given the basic properties of integrals, we have #A = color(red)(int_ln3^5 xe^x dx) - 3(color(blue)(int_0^5 x dx - int_0^ln 3 xdx))# Let #u = e^x => x = ln u#. #(du)/dx = e^x => dx = (du)/e^x = (du)/u# #int_ln3^5 xe^x dx = int_ln3^5 ln(u)*u (du)/u# #= int_ln3^5 ln u du# We will solve this with integration by parts : #int_a^b f(u)g'(u) = f(u)g(u)|_a^b - int_a^b g(u)f'(u)# In our case, #f(u) = ln(u) => f'(u) = 1/u# and #g'(u) = 1 => g(u) = u#, #b = 5# and #a=ln 3#. #int_ln3^5 ln u du = 5ln(5) - ln(3)ln(ln 3) - int_ln3^5 u*1/u du# #= 5ln(5) - ln(3)ln(ln3) - int_0^5 1 du + int_0^ln 3 1 du# #= 5ln(5) - ln(3)ln(ln3) - 5 + ln3# #color(red)(= 5(ln5 - 1) - ln(3)(ln(ln3) - 1))# We know #int_0^k x dx = k^2/2#, so #color(blue)(int_0^5 x dx - int_0^ln 3 xdx = (25 - ln^2 3)/2)# And finally, #color(purple) (A) = color(red)(5(ln5 - 1) - ln(3)(ln(ln3) - 1)) + color(blue)((3ln^2 3 - 75)/2)#

Christopher Agresta · Answer

undefined To find the net area between $ f(x) = xe^x - 3x $ and the x-axis over $ x $ in the interval $[1, 5]$, you need to compute the definite integral of $ |f(x)| $ over the interval $[1, 5]$. This is because the area between a curve and the x-axis can be negative if the curve dips below the x-axis.

So, the net area is given by:

$$
\text{Net Area} = \int_{1}^{5} |f(x)| \, dx
$$

Evaluate this integral to find the net area.