Given angle A and two sides, find Angle B.

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In summary, to find angle B in triangle ABC, where A=65 degrees, b=9, and a=10, use the law of sines to find the height h, then use the law of sines again to find angle B using arcsin(B). This can be simplified to the equation \frac{sin A}{a}=\frac{sin B}{b}.
  • #1
JacklinH.
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Homework Statement



In triangle ABC, A=65 degrees, b=9, and a=10. Find B.

Homework Equations


The Attempt at a Solution



I honestly don't know how to start this problem... I tried to use the law of cosines to find side C, and then again to find Angle B, but that answer was incorrect. I haven't been in Trig for two years, and can't think of how else to start it.

Thanks for any help!
 
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  • #2
Try the law of sines.
 
  • #3
6joz60.jpg


First use the sine to find h, and then use again the sine to find B, using arcsin(B).

[tex]\sin A = \frac{h}{b}\text{ and } \sin B = \frac{h}{a}.[/tex]

Regards.
 
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  • #4
Дьявол said:
6joz60.jpg


First use the law of sine to find h, and then use again the law of sine to find B, using arcsin(B).

[tex]\sin A = \frac{h}{b}\text{ and } \sin B = \frac{h}{a}.[/tex]

Regards.
so that h= b sin A= a sin B and, as a result, [itex]\frac{sin A}{a}= \frac{sin B}{b}[/itex]. That is the "sine law" Bohrok is talking about.
 
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  • #5
HallsofIvy said:
so that h= b sin A= a sin B and, as a result, \(\displaystyle \frac{sin A}{a}= \frac{sin B}{b}\). That is the "sine law" Bohrok is talking about.
Yes, you're right. I misspelled the words. I thought of "sine" and not "sine law".

Anyway, the things are same, and he would come up with the same result.
 
  • #6
Yes, I wasn't criticizing, just pointing out to JacklinH that you were both giving the same advice!
 

FAQ: Given angle A and two sides, find Angle B.

How do I find Angle B if I am given angle A and two sides?

If you are given angle A and two sides, you can use the Law of Cosines to find Angle B. This law states that c² = a² + b² - 2abcosC, where c is the side opposite angle C. Therefore, you can rearrange the equation to find cosC = (a² + b² - c²) / 2ab. Then, you can use the inverse cosine function to find the measure of angle C, which is also equal to Angle B.

What is the Law of Cosines and how does it help find Angle B?

The Law of Cosines is a mathematical formula used to find the missing side or angle of a triangle when given two sides and the included angle. This law is particularly useful when dealing with non-right triangles, as the Pythagorean Theorem only applies to right triangles. By using the Law of Cosines, you can find the measure of Angle B by rearranging the formula and solving for the missing angle.

Can I use the Law of Sines to find Angle B instead?

Yes, you can also use the Law of Sines to find Angle B. This law states that sinA / a = sinB / b = sinC / c, where A, B, and C are angles and a, b, and c are the corresponding sides. To find Angle B, you can use the ratio sinB / b = sinC / c and solve for B using the given values. However, the Law of Sines only applies when you are given an angle and its opposite side, or two angles and their corresponding sides.

What if I am given two angles and one side, can I still find Angle B?

Yes, you can use the Triangle Sum Theorem to find Angle B in this case. This theorem states that the sum of the interior angles of a triangle is always 180 degrees. Therefore, you can subtract the given angles from 180 degrees to find the measure of Angle B. Keep in mind that this method only works for triangles, as the sum of angles in other polygons may differ.

Are there any other methods to find Angle B?

Yes, there are various other methods to find Angle B depending on the given information. Some other methods include using the Law of Tangents, the Law of Cosines and Sines for oblique triangles, or using trigonometric identities and equations. It is important to carefully analyze the given information and choose the appropriate method to find Angle B accurately.

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