Question

PLEASE HELP!!! I don’t understand

Answer 1

Answer:

Step-by-step explanation:

Use the half angle identity for cosine:

cos(x/2)=+ or - sqrt(1+cos(x))/sqrt(2)

I'm going to figure out the sign part first for cos(x/2)...

so x is in third quadrant which puts x between 180 and 270

if we half x, x/2 this puts us between 90 and 135 (that's the second quadrant)

cosine is negative in the second quadrant

so we know that

cos(x/2)=-sqrt(1+cos(x))/sqrt(2)

Now we need cos(x)... since we are in the third quadrant cos(x) is negative...

If you draw a reference triangle sin(x)=3/5 you should see that cos(x)=4/5 ... but again cos(x)=-4/5 since we are in the third quadrant.

So let's plug it in:

cos(x/2)=-sqrt(1+4/5)/sqrt(2)

No one likes compound fractions (mini-fractions inside bigger fractions)

Multiply top and bottom inside the square roots by 5.

cos(x/2)=-sqrt(5+4)/sqrt(10)

cos(x/2)=-sqrt(9)/sqrt(10)

cos(x/2)=-3/sqrt(10)

Rationalize the denominator

cos(x/2)=-3sqrt(10)/10