What an integral

Calculus Level pending

d x c o s 3 x = γ ψ ( s i n x c o s 2 x + l n ( α + s i n x c o s x ) ) \int { \frac { dx }{ { cos }^{ 3 }x } } =\frac { \gamma }{ \psi } \left( \frac { sinx }{ { cos }^{ 2 }x } +ln\left( \frac { \alpha +sinx }{ cosx } \right) \right)

Where γ + ψ \gamma +\psi are co-prime find

γ + ψ + α 1 ? \gamma +\psi +\alpha-1 ?


The answer is 3.

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2 solutions

Trevor B.
Apr 27, 2014

The integral is the famous integral of secant cubed .

The integral can be done by parts, using this formula. u d v = u v v d u \int u\text{ }dv=uv-\int v\text{ }du

Let u = sec x , u=\sec x, d u = sec x tan x d x , du=\sec x\tan x\text{ }dx, v = tan x , v=\tan x, and d v = sec 2 x d x . dv=\sec^2x\text{ }dx. Plugging in to the formula, sec 3 x d x = sec x tan x sec x tan 2 x d x = sec x tan x sec x ( sec 2 x 1 ) d x = sec x tan x sec 3 x d x + sec x d x \begin{aligned} \int\sec^3x\text{ }dx&=\sec x\tan x-\int \sec x\tan^2x\text{ }dx\\ &=\sec x\tan x-\int\sec x(\sec^2x-1)\text{ }dx\\ &=\sec x\tan x-\int\sec^3x\text{ }dx+\int\sec x\text{ }dx \end{aligned} Move the integral of sec 3 x \sec^3x back over to the LHS. 2 sec 3 x d x = sec x tan x + sec x d x = sec x tan x + ln sec x + tan x + C \begin{aligned} 2\int\sec^3x\text{ }dx&=\sec x\tan x+\int\sec x\text{ }dx\\ &=\sec x\tan x+\ln|\sec x+\tan x|+C \end{aligned} Divide each side by 2 2 to get the final answer. sec 3 x d x = 1 2 ( sec x tan x + ln sec x + tan x ) + C \int\sec^3x\text{ }dx=\dfrac{1}{2}(\sec x\tan x+\ln|\sec x+\tan x|)+C

Rearranging of the trigonometric functions shows this. 1 2 ( sec x tan x + ln sec x + tan x ) + C = 1 2 ( sin x cos 2 x + ln ( 1 + sin x cos x ) ) + C \dfrac{1}{2}(\sec x\tan x+\ln|\sec x+\tan x|)+C=\dfrac{1}{2}\left(\dfrac{\sin x}{\cos^2x}+\ln\left(\dfrac{1+\sin x}{\cos x}\right)\right)+C Thus, γ = 1 , \gamma=1, ψ = 2 , \psi=2, and α = 1 , \alpha=1, so γ + ψ + α 1 = 3 \gamma+\psi+\alpha-1=\boxed{3}

Beakal Tiliksew
Apr 29, 2014

Sorry for the incorrect answer @Trevor B. .. Thats embarrassing careless me, how did you know the incorrect answer was 3 any ways This problem can also be solved with out integration by parts let u = c o s x u=cosx , and s = s i n x s=sinx = 1 u 3 = u u 4 = u ( 1 s 2 ) 2 = 1 4 u ( 1 1 + s + 1 1 s ) 2 = 1 4 ( u ( 1 + s ) 2 + u ( 1 s ) 2 ) + 1 2 u ( 1 s ) 2 = 1 4 ( 1 1 + s + 1 1 s ) + 1 4 ( u 1 + s + u 1 s ) = s 2 u 2 + 1 4 l n ( 1 + s 1 s ) = 1 2 ( s u 2 + l n ( 1 + s u ) ) = 1 2 ( s i n x c o s 2 x + l n ( 1 + s i n x c o s x ) ) =\int { \frac { 1 }{ { u }^{ 3 } } } \\ =\int { \frac { { u } }{ { u }^{ 4 } } } \\ =\int { \frac { u }{ { \left( 1-{ s }^{ 2 } \right) }^{ 2 } } } \\ =\int { \frac { 1 }{ 4 } u{ \left( \frac { 1 }{ 1+s } +\frac { 1 }{ 1-s } \right) }^{ 2 } } \\ =\frac { 1 }{ 4 } \int { \left( \frac { u }{ { \left( 1+s \right) }^{ 2 } } +\frac { u }{ { \left( 1-s \right) }^{ 2 } } \right) } +\frac { 1 }{ 2 } \int { \frac { u }{ { \left( 1-s \right) }^{ 2 } } } \\ =\frac { 1 }{ 4 } \left( \frac { -1 }{ 1+s } +\frac { 1 }{ 1-s } \right) +\frac { 1 }{ 4 } \int { \left( \frac { u }{ 1+s } +\frac { u }{ 1-s } \right) } \\ =\frac { s }{ 2{ u }^{ 2 } } +\frac { 1 }{ 4 } ln\left( \frac { 1+s }{ 1-s } \right) \\ =\frac { 1 }{ 2 } \left( \frac { s }{ { u }^{ 2 } } +ln\left( \frac { 1+s }{ u } \right) \right) \\ =\frac { 1 }{ 2 } \left( \frac { sinx }{ { cos }^{ 2 }x } +ln\left( \frac { 1+sinx }{ cosx } \right) \right)

You also need a + C +C in the problem statement.

Trevor B. - 7 years, 1 month ago

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haha, you remind me of my math teacher

Beakal Tiliksew - 7 years, 1 month ago

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Well, you do need it. If you search "The Importance of + C +C " you will find a note I wrote explaining why forgetting it can be a very bad mistake.

Trevor B. - 7 years, 1 month ago

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@Trevor B. i know , especially in physics, i tend to ignore them when there aren't necessary, not that i am correct though.

Beakal Tiliksew - 7 years, 1 month ago

I sent the clarification (1+2+1=4) :P

jatin yadav - 7 years, 1 month ago

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sorry about that, i should me more careful about it.

Beakal Tiliksew - 7 years, 1 month ago

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