Medium INTEGER +4 / -1 PYQ · JEE Mains 2020

If the system of equations
x - 2y + 3z = 9
2x + y + z = b
x - 7y + az = 24,
has infinitely many solutions, then a - b is equal to.........

Answer (integer) 5

Solution

D = 0<br><br>$$\left| {\matrix{ 1 &amp; { - 2} &amp; 3 \cr 2 &amp; 1 &amp; 1 \cr 1 &amp; { - 7} &amp; a \cr } } \right| = 0$$<br><br>$1(a + 7) + 2(2a - 1) + 3( - 14 - 1) = 0$<br><br>$a + 7 + 4a - 2 - 45 = 0$<br><br>$5a = 40$<br><br>$a = 8$<br><br>$${D_1} = \left| {\matrix{ 9 &amp; { - 2} &amp; 3 \cr b &amp; 1 &amp; 1 \cr {24} &amp; { - 7} &amp; 8 \cr } } \right| = 0$$<br><br>$\Rightarrow 9(8 + 7) + 2(8b - 24) + 3( - 7b - 24) = 0$<br><br>$\Rightarrow 135 + 16b - 48 - 21b - 72 = 0$<br><br>$\Rightarrow$ $15 = 5b$ <br><br>$\Rightarrow b = 3$<br><br>$a - b = 5$

About this question

Subject: Mathematics · Chapter: Matrices and Determinants · Topic: Types of Matrices and Operations

This question is part of PrepWiser's free JEE Main question bank. 274 more solved questions on Matrices and Determinants are available — start with the harder ones if your accuracy is >70%.

Drill 25 more like these. Every day. Free.

PrepWiser turns these solved questions into a daily practice loop. Chapter-wise drills, full mocks, AI doubt chat. No auto-renew.

Start free →