Need yet more help. This time, its differentiating trig, and a bit of trig algebra…

Have the curve y = 3xsin2x

Show that at the stationary points of the curve tan2x = -2x

dy/dx of the curve I got as 3sin3x + 6xcos2x

So I was thinking something along the lines of tan2x + 2x = 3sin2x + 6xcos2x but do not think that's right as its horrible to work with. Pretty confident of the dy/dx though.

Any ideas?

Remember, sinx/cosx = tanx.

Use the R alfa method to get tan you can then make tan = 0 which will prove its stationary. I'm not doing your homework for you!

R alfa? What?

3sin2x + 6xcos2x = 0 divide through by cos2x –>
3tan2x + 6x = 0
tan2x + 2x = 0

you'd already done the hard bit 🙂

Ah great cheers, almost had that, just had to divide by 3.. 🙄

You've done the hard bit already (aside from the sin3x -> sin2x typo). All you need to do is apply the fact that stationary point is code for "dy/dx = 0", substitute in your expression for dy/dx, and rearrange.

Edit: bah too slow!