Question

In triangle ABC, a = 3, b = 5, and c = 7. Find the approximate value of angle A.

Answer 1

Hint: cosine rule!

Answer 2

oh ok use the quadratic formula

Answer 3

22°
38°
142°
158°
these are the answer choices

Answer 4

\[\frac{ -b+-\sqrt{(b)-4(a)(c)} }{ ?2(a) }\]

Answer 5

plug it in

Answer 6

ok one second

Answer 7

im still not understanding

Answer 8

In geometry, it always helps to draw a diagram according to the given information.

Answer 9

cosine rule says:
\(a^2=b^2+c^2-2(b)(c) cos(A)\)
from which you can solve for cos(A):
\(\Large cos(A)=\frac{b^2+c^2-a^2}{2bc}\)
So you can substitute a,b,c into the equation and solve for angle A.

Answer 10

and what do you get when you that because i keepp getting something different

Answer 11

@kyrabaaker

Answer 12

@jcwilliams504 What have you done so far?

Answer 13

i plugged in everything but i dont know how to solve

Answer 14

Do you know the values of a, b, and c?

Answer 15

3,5, and 7

Answer 16

Good, so what did you get for:
\(\Large \frac{b^2+c^2-a^2}{2bc}\)

Answer 17

65/70 :/

Answer 18

@mathmate

Answer 19

That's correct.
You can find the angle A by solving
cos(A)=65/70=13/14
or
A = cos\(^{-1}\)(13/14)

Answer 20

whats after that?