Solve the equation secx=1+tanxsecx=1+tanx?

3 Answers
Jan 25, 2017

x=2npix=2nπ or x=2npi+pi/2x=2nπ+π2

Explanation:

secx=1+tanxsecx=1+tanx can be written as

1/cosx=1+sinx/cosx1cosx=1+sinxcosx

and assuming cosx!=0cosx0 this

1=cosx+sinx1=cosx+sinx

or sqrt2(sinxcos(pi/4)+cosxsin(pi/4))=12(sinxcos(π4)+cosxsin(π4))=1

or sqrt2sin(x+pi/4)=12sin(x+π4)=1

or sin(x+pi/4)=1/sqrt2=sin(pi/4)sin(x+π4)=12=sin(π4)

Hence x+pi/4=npi+(-1)^n(pi/4)x+π4=nπ+(1)n(π4)

or x=npi+(-1)^n(pi/4)-pi/4x=nπ+(1)n(π4)π4

which simplifies to x=2npix=2nπ or x=2npi+pi/2x=2nπ+π2

Equation given

secx=1+tanxsecx=1+tanx

=>secx-tanx=1.....(1)

Again we know

sec^2x-tan^2x=1....(2)

Dividing (2) by (1) we get

secx+tanx=1....(3)

Adding (1) and (3) we get

2secx =2

=>cosx=1

=>x=2npi" where " n in ZZ

Putting cosx=1 in equation (1) we get
tanx=0

=>x=npi" where " n in ZZ

but for n odd cosx=cosnpi=-1

So the only solution is x=2npi

Jun 12, 2018

x = 2kpi

Explanation:

1/(cos x) = 1 + sin x/(cos x)
1 = sin x + cos x (condition cos x != 0)
sin x + cos x = sqrt2cos (x - pi/4) = 1
cos (x - pi/4) = 1/sqrt2 = sqrt2/2
Trig table and unit circle give 2 solutions for (x - pi/4)-->
x - pi/4 = +- pi/4
a. x - pi/4 = pi/4 + 2kpi
x = pi/4 + pi/4 = pi/2 + 2kpi
This answer is rejected because of the above condition:
(cos x != 0 --> x != pi/2, and x != (3pi)/2)
b. x - pi/4 = - pi/4 + 2kpi
x = 2kpi
Check.
x = 2pi --< cos x = 1 --> sec x = 1/1 = 1 --> tan x = 0
1 + tan x = 1 + 0 = 1. Proved.