[SOLVED] Getting the wrong sign for cosx+sinx=rsin(x+alpha)

The question asks me to solve

So I put it into the form :

Then I equate the coefficients:

of cos x: [1]

of sinx: [2]

Divide [1] by [2] to get:

Square and add [1] and [2] to get:

Except that last statement doesn't seem to be true, according to

the answers in my book and when I have a look at the two curves in a

graphing application. I tried in the graphing app and

that seems to be correct...

but why??

What have I done wrong? Why am I getting a positive value for

alpha when I should be getting the same number but negative?

The only way I can see to get -49.4 would be by introducing a minus

sign when dividing [1] by [2]. But I don't see how to justify that.

Have I made a simple error? I can't spot it (Shake)...

Any help would be much appreciated, thanks in advance.