Q. Find the average value of the function f(x)=9−4x3 from x=3 to x=6. Express your answer as a constant times ln5.Answer: □ln5
Understand average value concept: Understand the concept of average value of a function. The average value of a function f(x) on the interval [a,b] is given by the formula: Average value = (b−a)1∫abf(x)dx Here, a=3 and b=6, so we need to integrate f(x)=(9−4x)3 from x=3 to x=6 and then multiply by the reciprocal of the interval's length.
Set up integral for average value: Set up the integral for the average value.Average value = (1/(6−3))×∫36(9−4x)3dxThis simplifies to:Average value = (1/3)×∫36(9−4x)3dx
Perform substitution to simplify integral: Perform a substitution to simplify the integral.Let u=9−4x, then du=−4dx or dx=−4du.When x=3, u=9−4(3)=9−12=−3.When x=6, u=9−4(6)=9−24=−15.Now we can rewrite the integral in terms of u:Average value = (1/3)×∫−3−15(−43)×(u1)du
Evaluate the integral: Evaluate the integral.The integral of u1 with respect to u is ln∣u∣, so we have:Average value = (31)⋅(4−3)⋅[ln∣u∣] from −3 to −15This simplifies to:Average value = (−41)⋅[ln∣−15∣−ln∣−3∣]Since ln∣a∣−ln∣b∣=ln∣ba∣, we can combine the logarithms:Average value = (−41)⋅ln(315)Average value = (−41)⋅ln(5)
Check for errors and write final answer: Check for math errors and write the final answer.We have correctly applied the rules of logarithms and the integral was evaluated correctly. The negative sign comes from the fact that du was −4dx, which we accounted for in the integral.Final answer: (−1/4)ln(5)