help?! (1 Viewer)

[_deloso]

 

[math man]

let u = sinx, then you can do it by substitution easy

 

[_deloso]

let u = sinx, then you can do it by substitution easy

i still dont get it

 

[math man]

let u= sinx,



we now sub sinx for u and cosxdx for du as follows:



which im sure you can finish

 

[D94]

u = sinx
du/dx = cosx
du = cosx dx
du = cosx dx

Substitute into the question to get: (u^2 * du)

Change the limits because you're integrating with respect to u now (du). Then apply a simple integration of u squared.

 

[_deloso]

let u= sinx,



we now sub sinx for u and cosxdx for du as follows:



which im sure you can finish

okay got it. thanks dude!!! the man!

 

[someth1ng]

You can even do this question straight from memory since d/dx[sin^3(x)]=3sin^2xcosx

 

[math man]

you can also integrate it straight away realsing that cosx is the derivate of sinx using the reverse power rule for integrals:



which is in the cambridge 3u book..but this is more a 4u formula

 

[barbernator]

u = sinx
du/dx = -cosx

The derivative of u=sinx is du/dx=cosx, NOT -cosx

 

[D94]

The derivative of u=sinx is du/dx=cosx, NOT -cosx

Yeah, my bad. Fixed.

OP, You can use the reverse chain rule method if you know it (ie. integrate with respect to sinx)

 

[barbernator]

you could draw accurate graphs on really small grid paper and count the number of squares underneath?

 

[math man]

you could draw accurate graphs on really small grid paper and count the number of squares underneath?

i hope this is a troll lol

 

[barbernator]

haha yeh

 

[someth1ng]

you could draw accurate graphs on really small grid paper and count the number of squares underneath?

LOL, I actually thought this was funny. How accurate is your counting?

 

[b3kh1t]

Yeah, my bad. Fixed.

OP, You can use the reverse chain rule method if you know it (ie. integrate with respect to sinx)

you can also integrate it straight away realsing that cosx is the derivate of sinx using the reverse power rule for integrals:



which is in the cambridge 3u book..but this is more a 4u formula

This is the general method for the students at the extension 2 level and it is usually not taught in 2 unit or extension 1 maths.

 

[artosis]

you could draw accurate graphs on really small grid paper and count the number of squares underneath?

i actually lol'd
good one

 

[artosis]

Actually, i know this is acceptable in 4u, ive seen heaps of solutions do it
but in 3u, if it doesnt ask for substitution specifically, are we allowed to quote this formula and use it?
/will it guarantee those 3 or 4 marks?

 

[b3kh1t]

Actually, i know this is acceptance in 4u, ive seen heaps of solutions do it
but in 3u, if it doesnt ask for substitution specifically, are we allowed to quote this formula and use it?
/will it guarantee those 3 or 4 marks?

You should be allowed, however it is very rare for them to give you a question in 3 unit where you must find what you need to substitute. But if you do, then you must show your working out, so state the formula and I believe that you should receive the marks.