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# lines a(1) and a(2 in two-dimensional space intersect at a point, forming an acute angle A

lines a(1) and a(2 in two-dimensional space intersect at a point, forming an acute angle A. the slope of a(1) is square root of 5 , and cos A= 2/9. What is the slope of a(2)? There are two answers, both must be given to properly answer the question

### 2 Answers by Expert Tutors

Robert J. | Certified High School AP Calculus and Physics TeacherCertified High School AP Calculus and Ph...
4.6 4.6 (13 lesson ratings) (13)
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tanθ1 = √5, tanA = √(77)/2 (since cosA = 2/9, and A is an acute angle.)

tanθ2 = m, where m is the slope to be found.
Since θ21 = ±A,
m = tan(θ1±A) = [tanθ1±tanA]/[1-+tanθ1*tanA]
Plugging in tanθ1 = √5, tanA = √(77)/2 gives you the exact final answers.

Answer in decimal form: m = -.199 or m = -.752.
Andre W. | Friendly tutor for ALL math and physics coursesFriendly tutor for ALL math and physics ...
5.0 5.0 (3 lesson ratings) (3)
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The relation between the slope of a line and the angle B it makes with the horizontal (x-axis) is
m = tan B.

You have two lines with slopes
m1 = tan B1 = √5
m2 = tan B2 = ?

The angle A between the two lines can be

A = B2 - B1
or
A = B1 - B2
depending on which line is steeper.
Therefore,
B2 = B1 + A
or
B2 = B1 - A

In your case, B1=tan-1√5=65.9° and A = cos-1(2/9)=77.2°, so that

B2 = tan-1√5 ± cos-1(2/9)
B2 = 143°=-36.9°
or
B2 = -11.3°
The slopes are
tan(-36.9)=-0.75=-3/4
and
tan(-11.3)=-0.2=-1/5