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# Calculus/Mathematics

Question
I was hoping you could help me with this math question. It was on last-night's homework and I got it wrong. I tried to ask my teacher today, but he is a substitute that wasn't able to explain it very well (my usual teacher is on maternity leave). If you could show me the answer, as well as how you get that answer, it would be very helpful. I'm a bit lost.

Using a double-angle formula, find the exact value for cos2u when sinu = 3/5 where pi/2 < u < pi

Thank you.

cos(2u) = 1- 2 (sin(u))^2 because cos(a+b)=cos(a)cos(b)-sin(a)sin(b) (and (cos(u))^2 = 1-(sin(u))^2)

So cos(2u) = 1- 2 * (sin(u))^2
= 1 - 2 * (3/5)^2
= 1 - 2 * (9/25)
= 1 - 18/25

I leave it to you to fill in the last step.
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Calculus

Volunteer

#### Amos

##### Expertise

I can answer all calculus question. I am a Math Lecturer and I teach Math in a College, usually Calculus 1, Calculus 2 and Calculus 3 and Linear Algebra.

##### Experience

I have been teaching in this college for more than 12 years. Prior to that, I taught for three years as Visiting Assistant Professor.

Education/Credentials
I got my doctorate from Univ. of Rochester in Algebraic Toppology. I got a MA from Univ of Rochester, an MA from York Univ. in Toronto and almost did my M.Sc in the National Univ. of Singapore.