Can someone explain me what happen in this step? Circuits 1

[sammyqw]

Homework Statement

http://imgur.com/tQR7Jdt

Homework Equations

xc= vm/im

The Attempt at a Solution

http://imgur.com/a/RwIUA

Can someone please explain where did they get that pi/2 ? did they integrate 2cos(10t)or something? Thanks

 

[gneill]

They changed the trig function used to describe the current from a sine function to a cosine function. This just involves a phase shift.

 

[sammyqw]

thank you i see now