1985 AHSME Problems/Problem 28


In $\triangle ABC$, we have $\angle C=3\angle A, a=27$ and $c=48$. What is $b$?

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$\mathrm{(A)\ } 33 \qquad \mathrm{(B) \ }35 \qquad \mathrm{(C) \  } 37 \qquad \mathrm{(D) \  } 39 \qquad \mathrm{(E) \  }\text{not uniquely determined}$

Solution 1

From the Law of Sines, we have $\frac{\sin(A)}{a}=\frac{\sin(C)}{c}$, or $\frac{\sin(A)}{27}=\frac{\sin(3A)}{48}\implies 9\sin(3A)=16\sin(A)$.

We now need to find an identity relating $\sin(3A)$ and $\sin(A)$. We have




Thus we have $9(3\sin(A)-4\sin^3(A))=27\sin(A)-36\sin^3(A)=16\sin(A)$

$\implies 36\sin^3(A)=11\sin(A)$.

Therefore, $\sin(A)=0, \frac{\sqrt{11}}{6},$ or $-\frac{\sqrt{11}}{6}$. Notice that we must have $0^\circ<A<45^\circ$ because otherwise $A+3A>180^\circ$. We can therefore disregard $\sin(A)=0$ because then $A=0$ and also we can disregard $\sin(A)=-\frac{\sqrt{11}}{6}$ because then $A$ would be in the third or fourth quadrants, much greater than the desired range.

Therefore, $\sin(A)=\frac{\sqrt{11}}{6}$, and $\cos(A)=\sqrt{1-\left(\frac{\sqrt{11}}{6}\right)^2}=\frac{5}{6}$. Going back to the Law of Sines, we have $\frac{\sin(A)}{27}=\frac{\sin(B)}{b}=\frac{\sin(\pi-3A-A)}{b}=\frac{\sin(4A)}{b}$.

We now need to find $\sin(4A)$.




Therefore, $\frac{\frac{\sqrt{11}}{6}}{27}=\frac{\frac{35\sqrt{11}}{162}}{b}\implies b=\frac{6\cdot27\cdot35}{162}=35, \boxed{\text{B}}$.

Solution 2

Let angle $A$ be equal to $x$ degrees. Then angle $C$ is equal to $3x$ degrees, and angle $B$ is equal to $180-4x$ degrees. Let $D$ be a point on side $AB$ such that $\angle ACD$ is equal to $x$ degrees. Because $2x+180-4x+\angle CDB=180$, angle $CDB$ is equal to $2x$ degrees. We can now see that triangles $CDB$ and $CDA$ are both isosceles, with $CB=DB$ and $AD=AC$. From isosceles triangle $CDB$, we now know that $BD = 27$, and since $AB = c = 48$, we know that $AD = 21$. From isosceles triangle $CDA$, we now know that $CD = 21$. Applying Stewart's Theorem on triangle $ABC$ gives us $AC = 35$, which is $\boxed{\text{B}}$.

See Also

1985 AHSME (ProblemsAnswer KeyResources)
Preceded by
Problem 27
Followed by
Problem 29
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