# Trig limit

1. Feb 6, 2007

### Rasine

lim x-> 7pi/4 (cosx+sinx)/(cos2x)

so what i was trying to do here was replace cos2x with the double angle id which is cos^2x-sin^2x

then i factor that and cancle out cosx+sinx

now i have 1/cosx-sinx...so i tried direct sub. and i get 1/sqroot2/2+squroot2/2 which i end up with sqroot 2 but that isn't the right answer

isn't cos 7(pi/4)= squroot 2/2 and sin 7(pi/4)= -squroot2/2?

please show me where i am going wrong

2. Feb 7, 2007

### dextercioby

You should end up with $\frac{1}{\sqrt{2}}=\frac{\sqrt{2}}{2}$.

3. Feb 7, 2007

### Gib Z

He means you did your algebra wrong btw.