Page 246 - ICSE Math 7

MENTAL MATHS

1. Choose the correct answer.

(a) A farmer wants to fence his rectangular field measuring 14 m by 10 m, one of whose lengths
is along the boundary wall of his house. What is the required length of the fence?
(i) 34 m (ii) 38 m (iii) 48 m (iv) 140 m
(b) If the base of a triangle is halved and its height is doubled, what will be the area of the
resulting triangle:

(i) increases (ii) decreases (iii) doubles (iv) remains the same
(c) The perimeter of a sector of a circle of radius 4 cm and formed by an arc of measure 11 cm is
____________.
(i) 15 cm (ii) 19 cm (iii) 30 cm (iv) 44 cm

(d) A paper is in the form of a rectangle of dimension
21 cm by 15 cm. A semicircle along its length is cut
off as shown. What is the area of the remaining part?
2
(i) 173.25 cm (ii) 163.25 cm 2 15 cm
2
(iii) 141.75 cm (iv) 315 cm 2
21 cm

(e) Two semicircles are cut off from a rectangle as shown.
What is the perimeter of the remaining part?

(i) 34 cm (ii) 68 cm 14 cm
(iii) 84 cm (iv) 128 cm

20 cm
(f) The area of a circular track whose outer and R = 5 m
inner radius is 5 m and 4 m is ____________.
2
(i) 41p m (ii) 25p m 2 r = 4 m
2
(iii) 16p m (iv) 9p m 2
2. Write True or False.

(a) A diagonal divides a parallelogram into two triangles of equal areas.
(b) The perimeter of a regular hexagon is 6l, where l is the length of its side.
(c) Two rectangles having equal areas should also have their perimeters equal.

(d) Two parallelograms on the same base and having equal perimeters have equal areas.
(e) For a given value of perimeter, a circle has the maximum area.
(f) If perimeter of a figure increases, then its area also increases.

(g) Area of a parallelogram can also be obtained by the product of its two adjacent sides.
(h) The perimeter of a semicircle of radius r is (2 + p)r.
22
(i) The exact value of p is or 3.14.
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