# Thread: trigonometry (1)

1. ## trigonometry (1)

Show that $sin^2x + \frac{1}{2}cos x = \frac{17}{16}-(\cos x-\frac{1}{4})^2$

I can do this part .

Hence , state the range

$\frac{1}{2\sin^2 x+\cos x-1}$ for $0\leq x\leq 2\pi$

Need help with this part . Thanks .

2. Hello thereddevils
Originally Posted by thereddevils
Show that $sin^2x + \frac{1}{2}cos x = \frac{17}{16}-(\cos x-\frac{1}{4})^2$

I can do this part .

Hence , state the range

$\frac{1}{2\sin^2 x+\cos x-1}$ for $0\leq x\leq 2\pi$

Need help with this part . Thanks .
$\frac{1}{2\sin^2 x+\cos x-1}= \frac{\frac12}{\sin^2x+\tfrac12\cos x - \tfrac12}$

$=\frac{\frac12}{\frac{17}{16}-(\cos x - \frac14)^2-\tfrac12}$

$=\frac{\frac12}{\frac{9}{16}-(\cos x - \frac14)^2}$

Now use the fact that $0\le(\cos x -\tfrac14)^2\le\tfrac{25}{16}$ to find the range of values of this expression. (Can you see why?)

3. Originally Posted by Grandad
Hello thereddevils $\frac{1}{2\sin^2 x+\cos x-1}= \frac{\frac12}{\sin^2x+\tfrac12\cos x - \tfrac12}$

$=\frac{\frac12}{\frac{17}{16}-(\cos x - \frac14)^2-\tfrac12}$

$=\frac{\frac12}{\frac{9}{16}-(\cos x - \frac14)^2}$

Now use the fact that $0\le(\cos x -\tfrac14)^2\le\tfrac{25}{16}$ to find the range of values of this expression. (Can you see why?)

Thanks Grandad , but i don see how you got that range ? Can you help me a little further

4. Hello thereddevils
Originally Posted by thereddevils
Thanks Grandad , but i don see how you got that range ? Can you help me a little further
The cosine of any angle lies between $-1$ and $+1$. So the maximum value of $(\cos x - \tfrac14)$ is $1-\tfrac14=\tfrac34$ and its minimum value is $-1-\tfrac14=-\tfrac54$.

But when we square it, $(\cos x - \tfrac14)^2$ is always positive, and now lies between $0$ and $(-\tfrac54)^2 = \tfrac{25}{16}$.

OK?