Cosine Rule - Angles intro SACHS
Cosine Rule - Angles
Step 1: To find an angle C,
you need all 3 sides a, b & c.
cos θ =
92 +        2 –         2
2 x 9 x
Step 2: Sub the sides intothe formula, make sure youare subtracting the 'opposite'side to your missing angle.
cos θ =
92 + 202 – 142
2 x 9 x 20
(Answer to 2 d.p., but keepall places on your screen)
Step 3: Input into your
calculator.
=
Step 4: Find θ by removingcos using cos–1.
cos θ = 0.79.....
θ = cos–1 0.79.....
θ =
(Answer correct to
the nearest degree)
o
cos θ =
16.52 +         2 –         2
2 x 16.5 x
Now, complete without
the prompts.
cos θ =
16.52 + 8.92 – 152
2 x 16.5 x 8.9
(Answer to 2 d.p., but keepall places on your screen)
=
cos θ = 0.43.....
θ =
(Answer correct to
the nearest degree)
o
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