Why Does My Capacitance Power Calculation Not Match the Given Answer?

[k_squared]

Homework Statement

The voltage across a capacitor with a value of 0.01 Farads is v(t) 100*cos(2pi60t). Find i(t),p(t),and w(t).

Homework Equations

i(t)=Cv'(t) [current]
p(t)=i(t)v(t) [power]
w= integral of p(t) [Energy stored over time]

The Attempt at a Solution

I was able to correctly determine the current of capacitance, -120*pi*sin(2pi60t) Amperes. However, this multiplied by the voltage, does NOT equal the given answer of -18,850sin(2pi120t) Watts.

Could someone please clear this up for me? I would've used proper symbols, but the test is quite shortly, no time to mess with that ... :/

Thanks!

 

[phyzguy]

Try using a trig identity for Sin(2X).