Trig limit

  • Thread starter Rasine
  • Start date
  • #1
Rasine
208
0
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
 

Answers and Replies

  • #2
dextercioby
Science Advisor
Homework Helper
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13,256
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You should end up with [itex] \frac{1}{\sqrt{2}}=\frac{\sqrt{2}}{2} [/itex].
 
  • #3
Gib Z
Homework Helper
3,352
6
He means you did your algebra wrong btw.
 

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