# Solutions to Math Problems (1)

Detailed solutions to the maths problems are presented.

 Solution to Problem 1: The circumference of a circle is given by C = 2 π r, where r is the radius of the circle. Substitute C by 72 π to obtain the equation 72 π = 2 π r Simplify and solve for r to obtain r = 36 Solution to Problem 2: Let L and W be the length and width of the garden. The statement "the length of a rectangular garden is 2 feet longer than 3 times its width" may be formulated by L = 2 + 3 W The formula for the perimeter is given by P = 2 L + 2 W Substitute P and L in the above equation by 100 and 2 + 3 W respectively to obtain 100 = 2(2 + 3 W) + 2 W Solve for W and L W = 12 and L = 2 + 3 W = 38. Check that the perimeter of the rectangular garden is 100 P = 2 L + 2 W = 76 + 24 = 100 Solution to Problem 3: Let x and y be the length and width of the field. The statement "a length 10 feet more than it is width" may be formulated by x = 10 + y The formula for the area is given by A = x y Substitute A and x in the above equation by 264 and 10 + y respectively to obtain 264 = (10 + y) y Write the above equation in standard form as follows y 2 + 10 y - 264 = 0 Solve the above equation for to obtain y = 12 and y = - 22 Since y is the value of a width it must be positive. The dimensions of the field are given by y = 12 and x = 10 + y = 22 As an exercise, check that the rectangular field has length 10 feet more than it is width and an area of 264. Solution to Problem 4: Let us use profit = revenue - costs to find a formula for the profit P P = ( x 2 + 100 x ) - ( 240 x + 500 ) Substitute P in the above equation by 10,000 dollars to obtain 10000 = x 2 - 140 x - 500 Write the equation in standard form and solve for x x 2 - 140 x - 10500 = 0 Solve the above equation for x x = 194.10 and x = -54.01 The number of units to produce must be positive, so x = 194. As an exercise, check that for the above value of x the profit is approximately (because of the rounding) equal to 10,000. Solution to Problem 5: Let x be the side of the smaller square and y be the side of the larger square. The statement "A square has a side 5 centimeters shorter than the side of a second square" may be formulated by x = y - 5 The area of the smaller square is equal to x 2 = (y - 5) 2 and the area of the larger square is equal to y 2 The statement "the area of the larger square is four times the area of the smaller square" may be formulated by y 2 = 4 (y - 5) 2 Write the above equation so that the right side is equal to 0 y 2 - 4 (y - 5) 2 = 0 The left side is made up of the difference of two squares and can be easily factored as follows [ y - 2 (y - 5) ] [ y + 2 (y - 5) ] = 0 Solve for y to find y = 10 and y = 10 / 3. We now use the equation x = y - 5 to find x y = 10 and x = 5 For the second solution y = 10 / 3, x is negative and cannot be accepted as the length of the side of a square must be positive. Solution to Problem 6: Let the two numbers be a and b and use the sum and product to write two equations with two unknowns a + b = 26 and a b = 165 Solve the first equation for b b = 26 - a Substitute b in the equation a b = 165 by 26 - a a (26 - a) = 165 Write the above equation in standard form - a 2 + 26 a - 165 = 0 Solve the above equation for a a = 11 and a = 15. Use b = 26 - a to find b when a = 11 , b = 15 and when a = 15 , b = 11. The two numbers are 11 and 15. Solution to Problem 7: Let x and y be the length and width of the rectangle. Using the formulas for the area and the perimeter, we can write two equations. 15 = x y and 16 = 2 x + 2 y Solve the second equation for x x = 8 - y Substitute x in the equation 15 = x y by 8 - y to rewrite the equation as 15 = (8 - y) y Solve for y to find y = 3 and y = 5 Use x = 8 - y to find x when y = 3 , x = 5 and when y = 5 , x = 3. The dimensions of the rectangle are 3 and 5. As an exercise, check that the perimeter of this rectangle is 16 and its area is 15. Solution to Problem 8: Let x and y be the two numbers such that x is larger than y. The statement "the larger number is four less than twice the smaller number" may be formulated by x = 2y - 4 We use the sum of the two numbers to write a second equation. x + y = 20 Substitute x by 2y - 4 in x + y = 20 to obtain 2y - 4 + y = 20 Solve for y to find y = 8 and x = 2y - 4 = 12 Solution to Problem 9: Let a and b be the two sides of the triangle such that a is longer than b. The statement "the hypotenuse of a right triangle is 2 centimeters more than the longer side of the triangle" may be fomulated by h = a + 2 or a = h - 2 The statement "the shorter side of the triangle is 7 centimeters less than the longer side" may be formulated . b = a - 7 or b = (h - 2) - 7 = h - 9 We now use Pythagora's theorem to write a third equation h 2 = a 2 + b 2 Substitute a by h - 2 and b by h - 9 in the above equation to obtain an equation in one variable only. h 2 = (h - 2) 2 + (h - 9) 2 Simplify and rewrite the above equation in standard form. h 2 -22 h + 85 = 0 Solve for h. h = 5 and h = 17. Only the solution h = 17 gives a and b positive and it is the length of the hypotenuse of the triangle. As an exercise, find a and b and see if a, b and h satisfies Pythagora's theorem. More math problems with detailed solutions in this site.