Chapter 7 - Applications of Trigonometry and Vectors - Section 7.3 The Law of Cosines - 7.3 Exercises - Page 320: 19

2.49 yards

Work Step by Step

We can use the law of cosines here because we know the lengths of two sides of the triangle and the measure of the included angle. The law of cosines is: $a^{2}=b^{2}+c^{2}-2bc\cos A$ where $b,c$ are the two known sides of the triangle while $A$ is the known angle. The unknown side opposite the known angle is $a$. Substituting the values in the formula and solving: $a^{2}=b^{2}+c^{2}-2bc\cos A$ $a^{2}=2.78^{2}+3.92^{2}-2(2.78)(3.92)\cos 41.4^{\circ}$ $a^{2}=7.73+15.37-22.50\cos 41.4^{\circ}$ $a^{2}=23.10-22.50\cos 41.4^{\circ}$ Using a calculator, we find that $\cos 41.4^{\circ}=0.75$. Therefore, $a^{2}=23.10-22.50\cos 41.4^{\circ}$ $a^{2}=23.10-22.50(0.75)$ $a^{2}=23.10-16.88$ $a^{2}=6.22$ $a=\sqrt {6.22}$ $a=2.49$ Therefore, the length of the unknown side of the triangle is 2.49 yards.

After you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.