1. finding an extremal

If , show that the extremal is

I've done:

Then using Euler's equation we get:

So

and then I'm stuck from here, I can't see how to turn this is into trigonometric functions? I tried letting which didn't work...

2. Solve the differential equation 1- y"= y which is the same as y"+ y= 1. That's a linear equation with constant coefficients. Its characteristic equation is $r^2+ 1= 0$ and I recommend trying y= A, a constant, for the "specific solution".