JEE MAIN - Mathematics (2020 - 8th January Evening Slot - No. 12)

The system of linear equations
$$\lambda $$x + 2y + 2z = 5
2$$\lambda $$x + 3y + 5z = 8
4x + $$\lambda $$y + 6z = 10 has
a unique solution when $$\lambda $$ = –8
no solution when $$\lambda $$ = 2
infinitely many solutions when $$\lambda $$ = 2
no solution when $$\lambda $$ = 8

Explanation

$$\Delta $$ = $$\left| {\matrix{ \lambda & 2 & 2 \cr {2\lambda } & 3 & 5 \cr 4 & \lambda & 6 \cr } } \right|$$

= $$\lambda $$ ( 18 – 5$$\lambda $$) – 2(12$$\lambda $$ – 20) + 2(2$$\lambda $$2 – 12)

= 18$$\lambda $$ – 5$$\lambda $$2 – 24$$\lambda $$ + 40 + 4$$\lambda $$2 – 24

= – $$\lambda $$2 – 6$$\lambda $$ + 16

= – ($$\lambda $$ + 8)($$\lambda $$ – 2)

For no solutions $$\lambda $$ = 0 $$ \Rightarrow $$ $$\lambda $$ = – 8, $$\lambda $$ = 2

when $$\lambda $$ = 2

$$\Delta $$x = $$\left| {\matrix{ 5 & 2 & 2 \cr 8 & 3 & 5 \cr {10} & 2 & 6 \cr } } \right|$$

= 5 (18 – 10) – 2 (48 – 50) + 2 (16 – 30)

= 40 + 4 – 28 $$ \ne $$ 0

So no solution for $$\lambda $$ = 2

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