Let the system of equations : 2x + 3y + 5z = 9 7x + 3y - 2z = 8 12x + 3y - (4 + )z = 16 - have infinitely many solutions. Then the radius of the circle centred at ( , ) and touching the line 4x = 3y is :
- 75
- 215
- 7
- 175
Answer
(1) 75
Solution
For infinitely many solutions, = 0:
= vmatrix 2 & 3 & 5 \ 7 & 3 & -2 \ 12 & 3 & -(4 + ) vmatrix
R2 R2 - R1, R3 R3 - R1:
= vmatrix 2 & 3 & 5 \ 5 & 0 & -7 \ 10 & 0 & -(9 + ) vmatrix = -3 [-5(9 + ) + 70 ] = -3(25 - 5 )
= 0 = 5
Then eq.(2) − eq.(1): 5x - 7z = -1; eq.(3) − eq.(1): 10x - 14z = 7 -
For consistency: 7 - = 2(-1) = 9
Centre (5, 9), line 4x - 3y = 0:
r = |4(5) - 3(9)| 42 + 32 = 75
Answer: (1)