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Thread: definite integrals

  1. #1
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    definite integrals

    can anyone please help me in finding the definite integral of y (6y-y^2)^1/2 from 0 to 6?
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  2. #2
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    Quote Originally Posted by sean123 View Post
    can anyone please help me in finding the definite integral of y (6y-y^2)^1/2 from 0 to 6?
    Dear sean123,

    I suppose your integral is,

    $\displaystyle \int_{0}^{6}{y(6y-y^2)^{\frac{1}{2}}dy}$

    Am I correct??
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  3. #3
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    $\displaystyle \int_0^6 y(6y- y^2)^{1/2} dy$

    The first thing I would do is "complete the square" inside the square root:
    $\displaystyle \int_0^6 y(9- (y- 3)^2)^{1/2}dy$

    Let u= y- 3. Of course, y= u+ 3 and when y= 0, u=-3, when y= 6, y= 3. This is now $\displaystyle \int_{-3}^3 (u+3)(9- u^2)^{1/2}du= u\int_{-3}^3 u(9- u^2)^{1/2}du+ 3\int_{-3}^3 (9- u^2)^{1/2}du$

    To integrate $\displaystyle \int_{-3}^3 u(9- u^2)^{1/2} du$ let $\displaystyle v= 9- u^2$.

    To integrate $\displaystyle 3\int_{-3}^3 (9- u^2)^{1/2}du$ let $\displaystyle u= 3 sin(\theta)$.
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