Question

In triangle $ABC$, $AB$ $=$$AC=15$ cm and $BC$ $=$ 18 cm, If cos $\angle ABC$ is $\frac{3}{m}$,

then $m$ is?

• A. $5$
• B. $4$
• C. $3$
• D. $1$

Answer 1

The correct option is A $5$

we know that, $AB=c=15,BC=a=18,CA=b=15$

now, $cos\angle ABC=cosB=\frac{c^2+a^2-b^2}{2ac}=\frac{{18}^2+{15}^2-{15}^2}{2\ast 18\ast 15}=\frac{18}{30}=\frac{3}{5}$

So, we can see that $m=5$

Therefore, Answer is $A$