00:00Hi guys, okay a real
00:02treat for you now. We
00:03are going to derive Euler's
00:05identity and this is kind
00:08of a great way to
00:09finish with the McLaurin series
00:12and actually this is my
00:13last lesson on calculus which
00:15is the last topic of
00:17the five so in a
00:19way this is my last
00:20lesson on the whole course
00:22and it's the perfect way
00:22to finish because this is
00:24the perfect equation. Now if
00:28you've
00:28never heard of orders identity.
00:30This is it, e to
00:30the i pi plus 1
00:31equals 0, and it is
00:33pretty much considered by everyone
00:36to be the most beautiful
00:38equation in mathematics. It's certainly
00:40one of the most famous.
00:41But if you look up,
00:42if you just Google the
00:43most beautiful equation in mathematics,
00:46that'll come up straight away
00:47without doubt. So, well, have
00:53a look at it. And
00:54it's certainly worth thinking, why
00:56is
00:56the beautiful equation. What's beautiful
00:57about it? Well, look, it's
00:59got look what's going on.
01:02It's got E. It's got
01:05I. It's got pi, which
01:07I mean, you might think
01:08what relation have they even
01:10got to each other? So
01:11it's got E. I pi.
01:13It's got one. It's got
01:14zero. These are some of
01:15these are like five of
01:16the most famous constants in
01:18mathematics. It's got a power,
01:20E to the power of
01:21this. It's got a multiplication.
01:23It's got a plus. It's
01:24got an
01:24equals it's it is pretty
01:26good now i'd heard about
01:31this equation and i knew
01:31it was like really famous
01:33and it was beautiful and
01:34i'm the first person might
01:38when my teacher said to
01:39me this is a beautiful
01:39equation i did look it
01:41in saffron off it's pretty
01:43cool but it wasn't until
01:45i actually understood how to
01:48get it myself that i
01:50really really appreciate it now
01:52If you have followed my
01:53lessons in order, and this
01:54is going to be your
01:55last lesson, you have all
01:56the skills required to derive
02:03it. And that's what we're
02:03going to do right now.
02:04And it comes at the
02:05end of the McLaren series
02:07sequence of lessons because we're
02:10going to use the McLaren
02:11series to derive it. Now
02:13note I've put over here
02:15on the right the McLaren
02:17series that are given in
02:18the formula booklet and we
02:19are going to use these
02:20in our derivation. So let
02:23us start with, I'm going
02:26to start with e to
02:28the x. Consider, e to
02:30the x, we have, e
02:32to the x is equal
02:34to, it's 1 plus, it's
02:371 plus x, I like
02:41that. Yeah, it's 1 plus
02:44x plus x squared over
02:48plus x cubed over three
02:52factorial plus x to the
02:54four over four factorial plus
02:57et cetera. Fine. Okay, now
03:01what if I get e
03:04to the ix? What happens?
03:06So I'm trying to get
03:06et i pi. So let's
03:07get et ix. I get
03:09e to the ix. What
03:12does this equal? Now it's
03:14one plus ix.
03:16plus i x squared over
03:222 factorial plus i x
03:26cubed over 3 factorial plus
03:29i x to the 4
03:33over 4 factorial by the
03:34way if you want to
03:36pause the video and try
03:37and see if you can
03:38try and figure this out
03:39by yourself and that would
03:41be better plus let's go
03:44dot dot dot. Okay. Now
03:47what happens here is I
03:49get this equals one plus
03:54i times x. Now what
03:56happens here is i squared
03:58becomes i squared is negative
04:01one. So this actually becomes
04:02minus x squared over two
04:07factorial. And then this i
04:10cubed is actually minus i.
04:12So this becomes minus i
04:14x cubed over three factorial.
04:19And then this minus i
04:20to the power of four
04:21is actually going to be
04:21plus. So this is going
04:23to be plus and i
04:26to the power of four
04:27is just going to be
04:27plus one. Because it's minus
04:30one, times minus one, which
04:31is plus one. So it's
04:32plus x to the four
04:34over four factorial. And then
04:37I'll keep going. I'll do
04:38a few more. It's going
04:39to be plus
04:40So again, it's gonna be
04:42plus, because it's gonna reach
04:44the power of five now,
04:45i to the power of
04:46five is just i, and
04:48then it starts repeating itself.
04:49So I minus one, minus
04:52i one, i, and then
04:54it's back to itself. So
04:57i, x to the five
05:01over five factorial, and then
05:04it's gonna be minus, I'll
05:06do one more, minus,
05:08X minus X to the
05:136 because now we're back
05:15here to minus X to
05:16the 6 over 6 factorial
05:18and then minus dot dot
05:21dot. Okay, great. Now, what
05:26I'm going to do, I'm
05:27going to take the real
05:29parts and I'm going to
05:30take the imaginary parts of
05:31the real parts of this
05:33expression are 1 minus
05:36x squared over two factorial
05:39plus x to the four
05:42over four factorial minus x
05:46to the six over six
05:47factorial. I can keep going.
05:50It would be plus x
05:52to the eight over eight
05:54factorial and then minus let's
05:57go again dot dot dot
05:59dot. But okay, so I've
06:02got I have a dot
06:03dot dot there and I'm
06:04actually up
06:04Put that in a bracket
06:05like this. And then it's
06:08plus, so now I'm gonna
06:10take the imaginary parts. I'm
06:11gonna have my i here,
06:14open bracket. So I'm gonna
06:16take out the i and
06:17what's left is x minus
06:20x cubed over three factorial
06:23plus x to the five
06:26over five factorial minus x
06:29to the seven. You can
06:30see a pattern here over
06:31seven factorial.
06:33plus dot dot dot close
06:39the bracket equals now look
06:43at this 1 minus x
06:45squared over 2 factorial plus
06:46x to the 4 over
06:464 factorial what is this
06:48the over here this is
06:51actually equal to cos x
06:55so this equals cos x
06:57and then this
07:01x minus x cubed over
07:02three factorial plus x to
07:03the five over five factorial
07:04is sin x. So this
07:07is now cos x plus
07:08i. There's an i there,
07:10i times sin x. So
07:13let's just bring this down
07:15as well e to the
07:17i x equals this. Okay.
07:19So this is an identity
07:20that we've used in complex
07:22numbers. If you've done complex
07:23numbers, you'd have seen this
07:24before. And this is where
07:27it comes from. Great.
07:29Now, we want e to
07:32the i pi, so that's
07:33sub in pi. I can
07:35say e to the i
07:37pi is equal to cos
07:40of pi plus i times
07:44sine of pi. Now we're
07:46nearly there. We are nearly
07:48there. Let me just come
07:50down a little bit here.
07:51e to the i pi,
07:53e to the i pi
07:54equals what's cos of
07:57Pi, well, cos of pi
08:00is negative 1. And what
08:05is sine of pi? What
08:07is 0? So e to
08:08the i pi equals negative
08:101. Or, let me change
08:12the color here. Make a
08:14big deal out of this.
08:17e to the i pi
08:18equals negative 1. Or, e
08:21to the i pi plus
08:241.
08:25equals zero. How neat is
08:29that, right? So that's, yeah,
08:34there we are. That's order's
08:36identity. That is the Maclaurin
08:41series. That is calculus. And
08:44again, depending when you finish
08:48this, this course or watch
08:50this lesson, that is the
08:52end of all
08:53the all my lessons in
08:56the um H .L. course
08:58so well done forget this
09:01far um when i say
09:04see you in the next
09:05lesson this time what of
09:06course i want you to
09:07be to be doing is
09:08going and practicing lots and
09:11lots of different types of
09:12questions on all these topics
09:13particularly the past papers so
09:16go and check out some
09:17of my videos on the
09:19on the on past papers
09:21you