7th Class
Mathematics
Mensuration
Question Bank
Mensuration
question_answer
DIRECTIONS: Passage ? 1 Read the passage(s) given below and answer the questions that follow Area between two concentric circles. Two circles are said to be concentric if they have a common center. Sometimes, we have to find the area between two concentric circles. For example, the area of a ring, the area of a circular field having a circular pond in the middle. So, let us consider two concentric circles with r as the radius of the inner circle and R as the radius of the outer circle. Area between the circles \[=\] Area of outer circle\[-\]Area of the inner circle \[=\pi {{R}^{2}}-\pi {{r}^{2}}=\pi ({{R}^{2}}-{{r}^{2}})\] A 7m wide path is to be constructed all around, and outside a circular garden of diameter 112m. The cost of constructing the path at Rs. 50 per square meter is
A)Rs. 13090
B)Rs. 130900
C)Rs. 1309
D)Rs. 13000
Correct Answer:
B
Solution :
Radius of inner circle =\[\frac{112}{2}\]= 56 m Radius of outer circle = 63 m Area of path = Area of outer circle ? Area of inner circle \[=\pi \left[ {{(63)}^{2}}-{{(56)}^{2}} \right]=\frac{22}{7}\times 119\times 7\] \[=2618\,{{m}^{2}}.\] Cost of constructing the path at Rs. per sq. m \[=~2618\times 50=Rs.\,\,130900\]