solve this algebraic equation.

The original problem has to do with the interest rate at which 2 choices have the same present value, but I figured out the equation on my own. For some reason, I can't figure out how to solve it, even though I'm usually very comfortable with solving algebraic equations...

6000 + (5940)/(1+i) =12000/[(1+i)^(1/2)]

The answer is i = .21 (answer in the back of the book, works when plugged in) but I can't figure out how to get it.

i was tryinhg to do this homework and i could not do it and it took me so much time.

If f(x) = sin x and f(a) = 2/6, then what is the value of f(a) + f(pi + a) + f(2pi + a)?