In triangle ABC, if a: B: C = 5:7:8, then angle B =?

Three sides respectively as, 5, 7, 8 and then cosine theorem, this is relatively simple, refueling!

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1. It is known that the lengths of three sides of △ ABC are a, B, C respectively, and satisfy a & # 178; + C & # 178; = 2Ab + 2bc-2b & # 178;. It is proved that △ ABC is an equilateral triangle
2. Given that a, B and C are the lengths of the three sides of △ ABC and satisfy A2 + 2B2 + c2-2b (a + C) = 0, then the shape of the triangle is______ ．
3. Given the trilateral length of △ ABC, a, B, C satisfies a & # 178; = B & # 178; + C & # 178; + BC, find ∠ a
4. It is known that the lengths of three sides of the triangle ABC are a, B, C respectively, and a & gt; C, then | C-A | - √ (AC-B) & # 178=
5. Given that a, B, C > 0 and 1 / (1 + a) + 1 / (1 + b) + 1 / (1 + C) = 1, we prove that ABC ≥ 8
6. If a, B, C > 0. ABC = 8
7. a. B, C ∈ R +, and (1 + a) (1 + b) (1 + C) = 8
8. Given a + B + C = 0, ABC = 8, 1 / A + 1 / B + 1 / C
9. Given a + B + C = 0, ABC = 8, it is proved that 1 / A + 1 / B + 1 / C is less than 0
10. Given a > 0, b > 0, and ABC = 1, prove (1 + a) (1 + b) (1 + C) 8
11. In the triangle ABC, the opposite sides of angles a, B and C are a, B and C respectively. If the square of a + the square of B = 25 and the square of a - the square of B = 7, find the height of the largest side
12. In △ ABC, if ∠ a - ∠ C = 25 ° and ∠ B - ∠ a = 10 °, then ∠ B=______ ．
13. In △ ABC, if A2 + B2 = 25, A2-B2 = 7, C = 5, then the height of the largest edge is______ ．
14. In △ ABC, three sides a, B and C satisfy a & # 178; + B & # 178; = 25, a & # 178; - B & # 178; = 7, C = 5, and find the height of the longest side
15. In △ ABC, the opposite sides of ∠ a, B and C are a, B and C respectively, and a & # 178; + 2Ab = C & # 178; + 2BC. Try to judge the shape of △ ABC As mentioned above~
16. Let a, B, C be the three sides of △ ABC, and (A & # 178; - 2Ab + B & # 178;) + (B & # 178; - 2BC + C & # 178;) = 0, which indicates the shape of △ ABC Urgent,
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19. Given a > 0, b > 0, 2C > A + B, prove (1) C ^ 2 > AB (2) C - √ C ^ 2-ab
20. Given a & 2 + B & 2 + C & 2-ab-3b-2c + 4 = 0, find the value of a + B + C