The values of m, n, for which the system of equations x + y + z = 4, 2x + 5y + 5z = 17, x + 2y + mz = n has infinitely many solutions, satisfy the equation :
- m2 + n2 - m - n = 46
- m2 + n2 + mn = 68
- m2 + n2 - mn = 39
- m2 + n2 + m + n = 64
Answer
(3) m2 + n2 - mn = 39
Solution
Infinitely many solutions = 0 and 1 = 2 = 3 = 0
= vmatrix 1 & 1 & 1 \ 2 & 5 & 5 \ 1 & 2 & m vmatrix = 3m - 6 = 0 m = 2
3 = vmatrix 1 & 1 & 4 \ 2 & 5 & 17 \ 1 & 2 & n vmatrix = 3n - 21 = 0 n = 7
(With m = 2, n = 7: 1 = 0 (two equal columns) and 2 = 0.)
m2 + n2 - mn = 4 + 49 - 14 = 39
Answer: (3)