Question

In a right angled triangle, if AB=3 cm, BC=4 cm then what is the value of cos A?

• A. $\frac{3}{5}$
• B. $\frac{4}{5}$
• C. $\frac{3}{4}$
• D. $\frac{2}{3}$

Answer 1

The correct option is A

$\frac{3}{5}$

Since ABC is a right-angled triangle, we can apply Pythagoras theorem to get the length of side AC (hypotenuse).
Therefore, $A{C}^2=A{B}^2+B{C}^2$
$\Rightarrow A{C}^2=3^2+4^2$
$\Rightarrow A{C}^2=9+16=25$
$\Rightarrow AC=\sqrt{2}5=5$
$\Rightarrow AC=5$
Now, $cosA=\frac{AB}{AC}=\frac{3}{5}$.