This Semis -- I Mean Seems Familiar

Calculus Level 1

0 2 x 1 ( x 1 ) 2 d x = ? \int_0^2 x \sqrt{1-(x-1)^2} \, dx = \, ?

Give your answer to 2 decimal places.


The answer is 1.57.

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2 solutions

First let u = x 1. u = x - 1. Then x = u + 1 , d u = d x x = u + 1, du = dx and as x : 0 2 x: 0 \rightarrow 2 we have u : 1 1. u: -1 \rightarrow 1. The integral then becomes

1 1 ( u + 1 ) 1 u 2 d u = 1 1 u 1 u 2 d u + 1 1 1 u 2 d u . \displaystyle\int_{-1}^{1} (u + 1)\sqrt{1 - u^{2}} du = \int_{-1}^{1} u\sqrt{1 - u^{2}} du + \int_{-1}^{1} \sqrt{1 - u^{2}} du.

Now the integrand of the first of these integrals is an odd function, and since we are integrating from 1 -1 to 1 , 1, this integral evaluates to 0. 0. Thus we just need to focus on the second of these integrals.

Letting u = sin ( θ ) , u = \sin(\theta), we have that d u = cos ( θ ) d θ du = \cos(\theta) d\theta and 1 u 2 = cos ( θ ) . \sqrt{1 - u^{2}} = \cos(\theta). The (indefinite) integral then becomes

cos 2 ( θ ) d θ = 1 2 ( 1 + cos ( 2 θ ) ) d θ = \displaystyle\int \cos^{2}(\theta) d\theta = \dfrac{1}{2} \int (1 + \cos(2\theta)) d\theta =

1 2 ( θ + sin ( θ ) cos ( θ ) ) = 1 2 ( arcsin ( u ) + u 1 u 2 ) , \dfrac{1}{2}(\theta + \sin(\theta)\cos(\theta)) = \dfrac{1}{2}(\arcsin(u) + u\sqrt{1 - u^{2}}),

which when evaluated from 1 -1 to 1 1 yields 1 2 ( ( arcsin ( 1 ) + 0 ) ( arcsin ( 1 ) + 0 ) ) = \dfrac{1}{2}((\arcsin(1) + 0) - (\arcsin(-1) + 0)) =

arcsin ( 1 ) = π 2 = 1.57 \arcsin(1) = \dfrac{\pi}{2} = \boxed{1.57} to 2 decimal places.

Moderator note:

You can simplify your second integral by considering the unit circle. Do you see why?

As the Challenge Master notes, the second integral represents the area of a unit semicircle, which has area π 2 . \frac{\pi}{2}.

Brian Charlesworth - 6 years ago

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Nice thought

Kumar Krish - 1 year, 12 months ago

Came to 1 2 ( 1 + cos ( 2 θ ) ) d θ \dfrac{1}{2} \int (1 + \cos(2\theta))d\theta and then only solved cos ( 2 θ ) d θ = 0 f o r g i v e n l i m i t s . \int \cos(2\theta)d\theta=0~ for ~given~ limits. and put the answer as 0!! as I often do.

Niranjan Khanderia - 5 years, 12 months ago

If you put it that way yes answer is pi/2 but if use arcsin(-1) =3pi/2 then the final answer would be -pi/2

Caeo Tan - 5 years, 3 months ago

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The range of the arcsin function is [ π 2 , π 2 ] [-\frac{\pi}{2}, \frac{\pi}{2}] , with arcsin ( 1 ) = π 2 \arcsin(-1) = -\frac{\pi}{2} , making the final answer 1 2 ( arcsin ( 1 ) arcsin ( 1 ) ) = π 2 \frac{1}{2}(\arcsin(1) - \arcsin(-1)) = \frac{\pi}{2} . Also note that the integrand is non-negative over the interval [ 0 , 2 ] [0,2] , so the value of the integral cannot be negative.

Brian Charlesworth - 5 years, 3 months ago

I know this is an almost unnecessary comment: But for anyone tempted to pick up that calculator on the final step.... no need! Just a reminder: Long division works on pi too..... 3.1415 / 2 = 1.57

Dustin Bryant - 3 years ago

my mistake: i thought arcsin 1 = 3 π / 4 \arcsin -1 = 3\pi /4 instead of π / 4 -\pi/4

Bostang Palaguna - 5 months, 2 weeks ago
Joe Potillor
Dec 1, 2016

Why odd function equals 0?

Max Fate - 3 years, 5 months ago

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This trick only works when the bounds of the integral are a a and a -a . The areas on the left hand and right side of such a graph cancel out. Consider sin x \sin x for example ... ?

Arthur Conmy - 3 years, 1 month ago

Why don't you first apply some techniques of definite integral and then solve indefinite It would be quite easier

Kumar Krish - 1 year, 12 months ago

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