Answer:
101
Explanation:
According to the Law of Cosines, for any △ABC with side lengths a, b, and c, a2=b2+c2−2bccosA; b2=a2+c2−2accosB; and c2=a2+b2−2abcosC
.
The Law of Cosines can be used because all three side lentghs are known.
Set up the equation for the Law of Cosines:
a2=b2+c2−2bccosA
Substitute the known values into the Law of Cosines:
312=222+182−2(22)(18)cosA
Square the values and multiply:
961=484+324−792cosA
Add:
961=808−792cosA
Subtract 808
from both sides:
153=−792cosA
Divide both sides by −792
:
153−792=cosA
According to this equation m∠A
is equal to the inverse cosine function of −153792
. Write this:
m∠A=cos−1(−153792)
Calculate the value of the inverse cosine −153792
on a calculator:
m∠A≈101°
Therefore, m∠A≈101°
.