Finding the maximum

JQ2009 · #1 $Old$ November 3rd, 2009, 10:22 AM

How would you find the maximum value and smallest +ve value the maximum occurs at for f(x) = -cosx + sqrt3sint ?

I've got as far as

f'(x) = sqrt3cosx + sinx

but I'm not sure where to go now?

Jameson · #2 $Old$ November 3rd, 2009, 12:55 PM

Quote:

Originally Posted by JQ2009 $View Post$

How would you find the maximum value and smallest +ve value the maximum occurs at for f(x) = -cosx + sqrt3sint ?

I've got as far as

f'(x) = sqrt3cosx + sinx

but I'm not sure where to go now?

Set f'=0, then combine the two trig terms into one. Solve for x to find test points for critical values.

HallsofIvy · #3 $Old$ November 4th, 2009, 03:24 AM

Quote:

Originally Posted by JQ2009 $View Post$

How would you find the maximum value and smallest +ve value the maximum occurs at for f(x) = -cosx + sqrt3sint ?

I've got as far as

f'(x) = sqrt3cosx + sinx

but I'm not sure where to go now?

Here's a completely different way to do that problem. A standard trig identity is cos(a+b)= cos(a)cos(b)- sin(a)sin(b). If we take b= x (I am assuming that is "sqrt 3 sin x", not "sqrt 3 sin t".) we have f(x)= (-1)cos x+ sqt(3) sin(x)= cos(a)cos(x)- sin(a)sin(x) so we would have to have cos(a)= -1 and sin(a)= -sqrt(3).

Well that's impossible because sqrt(3)> 1 and we would have to have $cos^2(a)+ sin^2(a)$ which is not true: $(-1)^2+ sqrt(3)^2= 4$ , not 1. So multiply and divide by the square root of that, 2:

$f(x)= -cos(x)+ \sqrt{3}sin(x)= 2(-(1/2)cos(x)+ \sqrt{3}/2 sin(x)$ and now we must have cos(a)= -1/2 and sin(a)= -sqrt(3)/2 which says that $a= -\pi/3$ .

That is, $f(x)= -cos(x)+ \sqrt{3}sin(x)= 2 cos(x- \pi/3)$ . And it is easy to find maximum and minimum values for that.