00:01
Okay, here i want to work out the volume and the green area here is rotated about the x -axis.
00:09
Now here we use the formula for rotation, which is this.
00:14
The volume is integral a to b, pi, y squared, dx.
00:23
That's rotation about the x axis from a to b.
00:28
So here we're doing two parts.
00:30
What i do first is i work out if i rotate this area under the curve y equals e power x over 2 about the x axis between these two limits here.
00:49
And then what i do is i will take away this volume here under the curve y equals e to minus x over 2.
00:59
And that's the answer.
01:04
So what we have then is this.
01:09
The required volume is the integral between ln6 and ln11, pi, the top one, this one squared.
01:23
So is e to the y over 2, 4 squared, minus the bottom one.
01:31
So integral, ln6, ln11, pi, e minus x over 2.
01:42
I'm not y there, x of course, and here squared.
01:55
How many dx here? i need dx here.
01:59
Well, this i can combine into a single integral.
02:03
It's going to be integral ln6 to ln11.
02:08
Take the pi outside.
02:11
I work at this, e to the x over 2 are squared.
02:15
Third law of exponents.
02:17
We multiply this by 2...