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Calculus Level 4

These are the graphs of sin x + cos x \sin x +\cos x and sin x cos x \sin x -\cos x

Let the area of every region enclosed by the two curves be A A

Find A 2 A^2

This is part of the Sinusoidal, Cosinusoidal series


The answer is 16.

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1 solution

Julian Poon
Oct 8, 2014

Finding the area nearest to the origin

hi hi

Finding intersection between the 2 functions \textbf{Finding intersection between the 2 functions}

(We are only interested in the positive area)

Solving s i n ( x ) + c o s ( x ) = s i n ( x ) c o s ( x ) sin(x)+cos(x)=sin(x)-cos(x) gives x = π 2 x=\frac { \pi }{ 2 } There is more than 1 solution but we are only taking this solution as it is the only one useful. . . . . Finding the intersect between the x-axis and the two functions \textbf{Finding the intersect between the x-axis and the two functions}

We are doing so to be able to integrate with the proper limits later.

So, using some geometry, we are again able to find the intersections. s i n ( x ) + c o s ( x ) = 0 sin(x)+cos(x)=0 We are only interested in the smallest negative solution. s i n ( x ) + c o s ( x ) = 0 , o p p o s i t e = a d j a c e n t sin(x)+cos(x)=0, opposite=-adjacent This is possible when ( x = π 4 x=-\frac{\pi}{4} ). . . Again, using some geometry, we can solve s i n ( x ) c o s ( x ) = 0 sin(x)-cos(x)=0 Here, we are only interested in the smallest positive solution. s i n ( x ) c o s ( x ) = 0 , o p p o s i t e = a d j a c e n t sin(x)-cos(x)=0, opposite=adjacent This is possible when x = π 4 x=\frac{\pi}{4} . . . . . Integrating the area \textbf{Integrating the area}

So, using the solutions above, putting in the proper limits, the area of the shaded figure above is [ π 4 π 2 s i n ( x ) + c o s ( x ) d x ] [ π 4 π 2 s i n ( x ) + c o s ( x ) d x ] = 2 \left[ \int _{ -\frac { \pi }{ 4 } }^{ \frac { \pi }{ 2 } }{ sin(x)+cos(x)dx } \right] -\left[ \int _{ \frac { \pi }{ 4 } }^{ \frac { \pi }{ 2 } }{ sin(x)+cos(x)dx } \right] =2 However, this is only half of the area we want. So, A = 2 × 2 = 4 A=2\times2=4 And since the question requires ( A 2 {A}^{2} ), A 2 = 4 2 = 16 {A}^{2}={4}^{2}=\boxed{16}

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