416 Higher Engineering Mathematics
If t=tan
θ
2
then cosθ=
1 −t^2
1 +t^2
and dx=
2dt
1 +t^2
from equations (2) and (3).
Thus
∫
dθ
5 +4cosθ
=
∫
(
2dt
1 +t^2
)
5 + 4
(
1 −t^2
1 +t^2
)
=
∫
(
2dt
1 +t^2
)
5 ( 1 +t^2 )+ 4 ( 1 −t^2 )
( 1 +t^2 )
= 2
∫
dt
t^2 + 9
= 2
∫
dt
t^2 + 32
= 2
(
1
3
tan−^1
t
3
)
+c,
from 12 of Table 40.1, page 399. Hence
∫
dθ
5 +4cosθ
=
2
3
tan−^1
(
1
3
tan
θ
2
)
+c
Now try the following exercise
Exercise 165 Further problems on the
t=tan
θ
2
substitution
Integratethefollowingwithrespect to thevariable:
1.
∫
dθ
1 +sinθ
⎡
⎢
⎣
− 2
1 +tan
θ
2
+c
⎤
⎥
⎦
2.
∫
dx
1 −cosx+sinx
⎡
⎢
⎣ln
⎧
⎪⎨
⎪⎩
tan
x
2
1 +tan
x
2
⎫
⎪⎬
⎪⎭
+c
⎤
⎥
⎦
3.
∫
dα
3 +2cosα
[
2
√
5
tan−^1
(
1
√
5
tan
α
2
)
+c
]
4.
∫
dx
3sinx−4cosx
⎡
⎢
⎣
1
5
ln
⎧
⎪⎨
⎪⎩
2tan
x
2
− 1
tan
x
2
+ 2
⎫
⎪⎬
⎪⎭
+c
⎤
⎥
⎦
42.3 Further worked problems on the
t=tan
θ
2
substitution
Problem 5. Determine
∫
dx
sinx+cosx
If tan
x
2
then sinx=
2 t
1 +t^2
,cosx=
1 −t^2
1 +t^2
and
dx=
2dt
1 +t^2
from equations (1), (2) and (3).
Thus
∫
dx
sinx+cosx
=
∫
2dt
1 +t^2
(
2 t
1 +t^2
)
+
(
1 −t^2
1 +t^2
)
=
∫
2dt
1 +t^2
2 t+ 1 −t^2
1 +t^2
=
∫
2dt
1 + 2 t−t^2
=
∫
−2dt
t^2 − 2 t− 1
=
∫
−2dt
(t− 1 )^2 − 2
=
∫
2dt
(
√
2 )^2 −(t− 1 )^2
= 2
[
1
2
√
2
ln
{√
2 +(t− 1 )
√
2 −(t− 1 )
}]
+c
(see Problem 11, Chapter 41, page 413),
i.e.
∫
dx
sinx+cosx
=
1
√
2
ln
⎧
⎪⎨
⎪⎩
√
2 − 1 +tan
x
2
√
2 + 1 −tan
x
2
⎫
⎪⎬
⎪⎭
+c
Problem 6.∫ Determine
dx
7 −3sinx+6cosx
From equations (1) and (3),
∫
dx
7 −3sinx+6cosx
=
∫ 2dt
1 +t^2
7 − 3
(
2 t
1 +t^2
)
+ 6
(
1 −t^2
1 +t^2
)