RD Chapter 8 Quadratic Equations Ex 8.7 Solutions
Question - 41 : - Find a natural number whose square diminished by 84 is equal to thrice of 8 more than the given number. [NCERT Exemplar]
Answer - 41 : -
Let n be a required natural number.
Square of a natural number diminished by 84 = n2 – 84
and thrice of 8 more than the natural number = 3 (n + 8)
Now, by given condition,
n2 – 84 = 3 (n + 8)
=> n2 – 84 = 3n + 24
=> n2 – 3n – 108 = 0
=> n2 – 12n + 9n – 108 = 0 [by splitting the middle term]
=> n (n – 12) + 9 (n – 12) = 0
=> (n – 12) (n + 9) = 0
=> n = 12 [n ≠ – 9 because n is a natural number]
Hence, the required natural number is 12.
Question - 42 : - A natural number when increased by 12 equals 160 times its reciprocal. Find the number. [NCERT Exemplar]
Answer - 42 : -
Let the natural number be x.
According to the question,
x + 12 = 160/x
On multiplying by x on both sides, we get
=> x2 + 12x – 160 = 0
=> x2 + (20x – 8x) – 160 = 0
=> x2 + 20x – 8x – 160 = 0 [by factorisation method]
=> x (x + 20) – 8 (x + 20) = 0
=> (x + 20) (x – 8) = 0
Now, x + 20 = 0 => x = -20 which is not possible because natural number is always greater than zero
and x – 8 = 0 => x = 8.
Hence, the required natural number is 8.