What is the value of cos(pi/9)cos((7pi)/(18))-sin(pi/9)sin((7pi)/(18)) ?

The value of cos(pi/9)cos((7pi)/(18))-sin(pi/9)sin((7pi)/(18)) is 0

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Popular >

cos(pi/9)cos((7pi)/(18))-sin(pi/9)sin((7pi)/(18))

Solution

cos (\frac{π}{9}) cos (\frac{7 π}{18}) - sin (\frac{π}{9}) sin (\frac{7 π}{18})

Solution

0

Solution steps

cos (\frac{π}{9}) cos (\frac{7 π}{18}) - sin (\frac{π}{9}) sin (\frac{7 π}{18})

Rewrite using trig identities:

cos (\frac{π}{2})

cos (\frac{π}{9}) cos (\frac{7 π}{18}) - sin (\frac{π}{9}) sin (\frac{7 π}{18})

Use the Angle Sum identity:

cos (s) cos (t) - sin (s) sin (t) = cos (s + t)

= cos (\frac{π}{9} + \frac{7 π}{18})

Simplify:

\frac{π}{9} + \frac{7 π}{18} = \frac{π}{2}

\frac{π}{9} + \frac{7 π}{18}

Least Common Multiplier of

9, 18 : 18

9, 18

Least Common Multiplier (LCM)

Prime factorization of

9 : 3 \cdot 3

9

9

divides by

39 = 3 \cdot 3

= 3 \cdot 3

Prime factorization of

18 : 2 \cdot 3 \cdot 3

18

18

divides by

218 = 9 \cdot 2

= 2 \cdot 9

9

divides by

39 = 3 \cdot 3

= 2 \cdot 3 \cdot 3

2, 3

are all prime numbers, therefore no further factorization is possible

= 2 \cdot 3 \cdot 3

Multiply each factor the greatest number of times it occurs in either

9

18

= 3 \cdot 3 \cdot 2

Multiply the numbers:

3 \cdot 3 \cdot 2 = 18

= 18

Adjust Fractions based on the LCM

Multiply each numerator by the same amount needed to multiply its

corresponding denominator to turn it into the LCM

18

For

\frac{π}{9} :

multiply the denominator and numerator by

2

\frac{π}{9} = \frac{π 2}{9 \cdot 2} = \frac{π 2}{18}

= \frac{π 2}{18} + \frac{7 π}{18}

Since the denominators are equal, combine the fractions:

\frac{a}{c} \pm \frac{b}{c} = \frac{a \pm b}{c}

= \frac{π 2 + 7 π}{18}

Add similar elements:

2 π + 7 π = 9 π

= \frac{9 π}{18}

Cancel the common factor:

9

= \frac{π}{2}

= cos (\frac{π}{2})

= cos (\frac{π}{2})

Use the following trivial identity:

cos (\frac{π}{2}) = 0

cos (\frac{π}{2})

cos (x)

periodicity table with

2 πn

cycle:

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

= 0

= 0

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Frequently Asked Questions (FAQ)

What is the value of cos(pi/9)cos((7pi)/(18))-sin(pi/9)sin((7pi)/(18)) ?
The value of cos(pi/9)cos((7pi)/(18))-sin(pi/9)sin((7pi)/(18)) is 0