The height of a cone is $$21$$cm. Find the area of the base if the slant height is $$28$$ cm.
Correct option is A. $$1078 cm^2$$.
We have,
$$h=21$$cm and $$l=28$$cm.
$$\therefore l^2=r^2+h^2$$
$$\Rightarrow r=\sqrt{l^2-h^2}=\sqrt{28^2-21^2}=7\sqrt{7} cm$$.
We know, the base of a cone is a circle.
$$\therefore$$ Area of the base$$=\pi r^2=\dfrac{22}{7}\times (7\sqrt{7})^2cm^2=1078cm^2$$.