Finding cosine C in triangle ABC with a = 3, B = 5 and C = 7

COSC = (a ^ 2 + B ^ 2-C ^ 2) / 2Ab = - 0.5 C = 120 degrees

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1. In △ ABC, if a = 7, B = 8, COSC = 1314, then the cosine of the largest angle is () A. −15B. −16C. −17D. −18
2. In the triangle ABC, the cosine of ∠ A is 5 / 13, and the cosine of ∠ B is 4 / 5?
3. In △ ABC, if a = 7, B = 8, COSC = 1314, then the cosine of the largest angle is () A. −15B. −16C. −17D. −18
4. In the triangle ABC, if a: B: C = 7:8:13, then the cosine of the largest inner angle in the triangle is
5. In △ ABC, if C = 60 ° and C2 = AB, then the shape of triangle is () A. Right triangle B. isosceles triangle C. equilateral triangle D. obtuse triangle
6. It is known that the three sides a, B and C of △ ABC satisfy A2 + B2 + C2 = 10A + 24B + 26c-338. Please judge the shape of △ ABC and explain the reason
7. In △ ABC, if B = 60 °, B & # 178; = AC, then C=
8. In △ ABC, B & # 178; = AC, B = 60 °, then a
9. In ABC, we know that B & # 178; - C & # 178; = A & # 178; + AC, then B =? A. 45 ° B, 120 ° C, 30 ° or 150 ° D, 60 ° or 120 ° C
10. As shown in the figure, in the triangle ABC, the angle a is equal to 60 degrees It is known that, as shown in the figure, in the triangle ABC, the angles a are equal to 60 degrees CE and BF are bisectors of the triangle ABC angles and intersect at point D Prove that De is equal to DF
11. In △ ABC, if Sina: SINB: sinc = 2:3:4, then the cosine value of the largest angle=______ ．
12. In △ ABC, if sin B = 2sinacosc and the cosine of the minimum angle is 3 / 4, (1) judge the shape of triangle ABC, (2) find the maximum angle of ABC
13. In △ ABC, the points o and E are on AB and AC respectively, ∠ DCB = ∠ EBC = 1,2 ∠ a, be and CD intersect at point O to prove BD = CE The following first answer is not for junior high school students, the second answer is wrong, also can't be used, hope someone continue to give guidance.
14. Known: as shown in the figure, in △ ABC, if ∠ A is an acute angle, points D and E are on AB and AC respectively, and ∠ DCB = ∠ EBC = 12 ∠ A. verification: BD = CE
15. Known: as shown in the figure, in △ ABC, if ∠ A is an acute angle, points D and E are on AB and AC respectively, and ∠ DCB = ∠ EBC = 12 ∠ A. verification: BD = CE
16. In the triangle ABC, the points D and E are on AB and AC, and the angle EBC = angle DCB = I / 2 angle a, BD = CE is proved
17. Known: as shown in the figure, in △ ABC, if ∠ A is an acute angle, points D and E are on AB and AC respectively, and ∠ DCB = ∠ EBC = 12 ∠ A. verification: BD = CE
18. In known triangle ABC, angle ACB is equal to 90 degrees, AC is equal to BC, ad is vertical to CE, be is vertical to CE, D and E are perpendicular feet
19. As shown in the figure, ABC is an equilateral triangle, BD is the middle line, extend BC to e, so that CE = CD
20. a. B and C are the three sides of triangle ABC. It is proved that a ^ 2 = B (B + C) is a = 2B Please give the proof of sufficiency and necessity