# Chapter 4 - Exponential and Logarithmic Functions - Section 4.1 Composite Functions - 4.1 Assess Your Understanding - Page 280: 70

$A[r(t)]=40000\ \pi \ t$

#### Work Step by Step

The area of a circle is given by the formula: $A=\pi r^2$ And we have the radius as a function of time: $r(t)=200\sqrt t.$ Therefore, the composite function will be: $A[r(t)]=\pi (200\sqrt t)^2=40000\ \pi \ t$

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