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

1. mathmate

Hint: cosine rule!

2. anonymous

oh ok use the quadratic formula

3. anonymous

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

4. anonymous

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

5. anonymous

plug it in

6. anonymous

ok one second

7. anonymous

im still not understanding

8. mathmate

In geometry, it always helps to draw a diagram according to the given information. |dw:1434459130639:dw|

9. mathmate

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.

10. anonymous

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

11. anonymous

@kyrabaaker

12. mathmate

@jcwilliams504 What have you done so far?

13. anonymous

i plugged in everything but i dont know how to solve

14. mathmate

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

15. anonymous

3,5, and 7

16. mathmate

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

17. anonymous

65/70 :/

18. anonymous

@mathmate

19. mathmate

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

20. anonymous

whats after that?