Questions With Answers

A set of trigonometry questions related to trigonometric functions are presented. The solutions and answers are provided.

## Question 1Find the exact value of sin (x / 2) if sin (x) = 1 / 4 and x is such that Pi / 2< x < Pi.
- To find sin (x/2), we use the following half angle formula
sin (x/2) = + or - SQRT [ (1 - cos x) / 2 ]
- Since Pi < x < Pi / 2 then Pi / 2 < x / 2< Pi / 4 so that x/2 is in quadrant 1 and sin (x/2) is positive. Hence
sin (x/2) = SQRT [ (1 - cos x) / 2 ]
- Given that sin (x) = 1 / 4, we use the trigonometric identity sin
^{2}x + cos^{2}x = 1 to find cos x, noting that x is in quadrant 2 and cos x is negative.
cos x = - sqrt(1 - sin^{2}x)
= - sqrt(1 - 1/16) = - sqrt(15) / 4
- We now substitute cos x by its value in the formula for sin (x/2).
sin (x/2) = SQRT [ (1 - sqrt(15) / 4) / 2 ]
- Which may be simplified to
= (1/4) SQRT [ 8 - 2 SQRT(15) ]
## Question 2x is in quadrant 3, approximate sin (2 x) if cos (x) = - 0.2. Round your answer to two decimal places.
- sin (2x) may be calculated using the double angle trigonometric identity
sin (2x) = 2 sin (x) cos (x)
- cos x is given, we need to find sin x using the identity sin
^{2}x + cos^{2}x = 1 and noting that x is in quadrant 3 where sin x is negative
sin x = - SQRT[ 1 - (- 0.2)^{2}]
- sin (2x) is now given by
sin (2x) = 2 [ - SQRT[ 1 - (- 0.2)^{2}] ] (-0.2)
= 0.39
## Question 3tan(x) = 4 and x is in quadrant III. Find the exact value of cos (x).
- We first use the pythagorean identity 1 + tan
^{2}x = sec^{2}x to find sec x in terms of tan x
sec (x) = + or - SQRT[ 1 + tan^{2}x ]
- Since x is in quadrant 3, sec (x) is negative. Hence
sec (x) = - SQRT[ 1 + 4^{2}]
= - SQRT (17)
- We now calculate cos (x) as follows
cos (x) = 1 / sec (x)
= - 1 / SQRT (17)
## Question 4cos (2x) = 0.6 and 2x is in quadrant I. Find the exact value of csc (x).
- We first use the identity cos(2x) = 2cos
^{2}x - 1 to find cos (x)
0.6 = 2cos^{2}x - 1
- Which gives
0.6 = 2cos^{2}x - 1
cos^{2}x = 0.8
- We now use the identity sin
^{2}x + cos^{2}x = 1 to find sin x
sin x = SQRT(1 - 0.8)
csc (x) = 1 / SQRT(1 - 0.8)
## Question 5Find the exact value of cos (15^{o}).
- Use the half angle formula cos (x/2) = + or - SQRT [ (1 + cos x) / 2 ] to write
cos (15^{o}) = SQRT [ (1 + cos 30^{o}) / 2 ]
= SQRT[ (1 + SQRT(3) / 2) / 2 ]
= (1/2) SQRT [ 2 + SQRT(3)]
## Question 6Find the exact value of tan (- 22.5^{o}).
- Use the identity for negative to write
tan (- 22.5^{o}) = - tan (22.5^{o})
- Use the half angle identity to tan (x/2) = sin x / (1 + cos x) to find tan(22.5) noting that 45/2 = 22.5
tan (22.5) = sin 45 / (1 + cos 45)
= ( SQRT(2) / 2 ) / [ 1 + SQRT(2)/2 ]
= SQRT(2) - 1
## Question 7x and y are angles in quadrant 1 and 3 respectively and cos (x) = a and sin (y) = b. Find cos (x + y) in terms of a and b.
- Use the sum formula cos (x + y) = cos x cos y - sin x sin y. We first need to find sin (x) and cos (y). x is in quadrant 1 where the sine is positive, Hence
sin (x) = SQRT (1 - a^{2})
- y is in quadrant 3 where the cosine is negative, hence
cos (y) = - SQRT(1 - b^{2}) - Finally
cos (x + y) = cos x cos y - sin x sin y
= - a SQRT(1 - b^{2}) - SQRT (1 - a^{2}) b
## Question 8x is an angle in quadrant 3 and sin (x) = 1 / 3. Find sin (3x) and cos (3x).
- Use the identity
sin(3x) = 3 sin x - 4 sin^{3}x
= 3 (1/3) - 4 (1 / 3)^{3}
= 23 / 27
## Question 9Reduce the power of the following trigonometric expression.4 sin ^{3}(x) + 4 cos ^{3}(x)
- Use the power reducing formulas sin
^{3}x = (3/4) sinx - (1/4) sin (3x) and cos^{3}x = (3/4) cos x+ (1/4) cos(3x) to write that
4 sin^{3}(x) + 4 cos^{3}(x)
= 4 [ (3/4) sinx - (1/4) sin (3x) ] + 4 [ (3/4) cos x + (1/4) cos(3x) ]
= 3 sinx - sin (3x) + 3 cos x + cos 3x
## Question 10Factor the following trigonometric expression.sin (x) + sin (2x)
- Use the identity sin (2x) = 2 sin x cos x to write
sin (x) + sin (2x) = sin x + 2 sin x cos x
= sin x (1 + 2 cos x)
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