# How do I manipulate this?

1. Dec 5, 2014

### shreddinglicks

1. The problem statement, all variables and given/known data
I just want to manipulate this equation

2. Relevant equations
8 - {[(4(rcos(x))^2 +4(rsin(x)^2)^2] / 2}

I should have:

8-8r^4

3. The attempt at a solution

I used FOIL and the Pythagorean identity to get:

8- {[16r^4 + (32r^4sin(x)^2cos(x)^2)] / 2}

which is wrong

2. Dec 5, 2014

### LCKurtz

Use the basic trig identity involving $\sin^2x$ and $\cos^2 x$ before expanding.

3. Dec 5, 2014

### Staff: Mentor

That's not an equation. The tipoff that it isn't an equation is that no = sign appears anywhere in this expression.

4. Dec 5, 2014

### shreddinglicks

Thanks, I got the answer. Much easier than what I did the 1st time.

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5. Dec 5, 2014

### shreddinglicks

It's part of an equation.

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40.1 KB
Views:
35
6. Dec 5, 2014

### Staff: Mentor

Not relevant.

What you said was "I just want to manipulate this equation." but what you showed is not an equation. That was my point. It's not a major thing, but it is important to get the terminology right so as to be able to communicate effectively.

7. Dec 5, 2014

### BiGyElLoWhAt

Also, I believe you typed your equation wrong.
you typed:
8 - {[(4(rcos(x))^2 +4(rsin(x)^2)^2] / 2}
I think you meant to type
8 - {[(4(rcos(x))^2 +4(rsin(x)^2)]^2 / 2}