In the adjoining figure the area enclosed between the concentric circl

1.	In the adjoining figure the area enclosed between the concentric circles is 770 cm square. lf the radius of the outer circle is 21 cm , calculate the radius of inner circle.
Answer» $\sf\large\underline{Given:}$ $\rm{\implies Difference\:area\:_{(outer\:circle-inner\:circle)}=770cm^2}$ $\rm{\implies Outer\:_{(radius\:of\:circle)}=21cm}$ $\sf\large\underline{To\: Find:}$ $\rm{\implies Inner\:_{(radius\:of\:circle)}=?}$ $\sf\large\underline{Solution:}$ $\tt{\implies Let,\:the\: inner\: radius\:of\:circle\:be\:r}$ $\sf\large\underline{Formula\:used:}$ $\tt{\implies Area\:_{(circle)}=\pi\:r^2}$ $\tt{\implies Outer\:_{(area)}-Inner\:_{(area)}=Difference\:_{(area)}}$ $\tt{\implies \dfrac{22}{7}\times\:21^2-\dfrac{22}{7}\times\:r^2=770}$ we TAKE COMMON here 22/7 here] $\tt{\implies \frac{22}{7}\bigg(21^2-r^2\bigg)=770}$ $\tt{\implies 441-r^2=770\div\dfrac{22}{7}}$ $\tt{\implies 441-r^2=770\times\dfrac{7}{22}}$ $\tt{\implies 441-r^2=245}$ $\tt{\implies -r^2=245-441}$ $\tt{\implies -r^2=-196}$ $\tt{\implies r^2=196}$ $\tt{\implies r=\sqrt{196}=14cm}$ $\bf\large{Hence,}$ $\rm{\implies Inner\:_{(radius\:of\:circle)}=14cm}$