trig question...what am i doing wrong?

👨🏻‍🏫 學術台數學

[匿名]

2009-05-04 4:55 pm

find angle BAD

AD=20cm
DB=18.4cm
DA=10cm

i get angle BAD as 66.2, yet the answer sheet says it is 66.4, what on earth am i doing wrong?

更新1:

hhmmm, could be

更新2:

haha, i know how to work it out, its just that the stupid mark scheme might be wrong.lol

回答 (5)

Christopher

2009-05-04 5:20 pm

✔ 最佳答案

There is a probability that the answer sheet is WRONG... Too bad it tries to trick you then...

...BUT the info "DB=18.4cm" really pricks my mind. Was it calculated from previous part of the question and rounded off to 1 d.p.?

If so, I've checked it out (haha~). If DB is actually 18.43 or 18.44 cm angle BAD will be 66.4`````````

👍 1 👎 0

[匿名]

2009-05-04 5:14 pm

Look at your given. It says AD = 20 cm and DA = 10 cm. I believe one of these is the real AD and the other one is AB.

Now, assuming AD = 20 cm and AB = 10 cm, then
Using the Cosine Law, we have
angle BAD = arccos({BD^2 - [(AD^2) + (AB^2)]}/[-2(AD)(AB)])
angle BAD = arccos({18.4^2 - (20^2) - (10^2)]}/[-2(20)(10)])
angle BAD = 66.2

You're right.

Hope this helps.

teddy boy

👍 0 👎 0

[匿名]

2009-05-04 5:09 pm

You have given the measurements of sides incorrectly

AD and DA are the same sides

But using 18.4cms as the side opposite the required angle
and the Cosine Rule I also get 66.19 degrees which is 66.2 to 2 decimal places

👍 0 👎 0

[匿名]

2009-05-04 5:07 pm

You wrote AD= 20cm, but DA=10cm..... or do you mean BA=10cm?

If BA=10cm;

Then you'd use the cosine formula giving BAD as 66.197= 66.2

I think either you misread the question or the marking scheme is wrong; since your answer is correct given the above information.

👍 0 👎 0

[匿名]

2009-05-04 5:01 pm

I get 66.2 too. Maybe the answer sheet is wrong.

👍 0 👎 0

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