That is close

Algebra Level 5

2 x 2 5 x + 3 4 x m \dfrac{2x^{2} - 5x +3}{4x - m}

How many integers m m satisfy the following property:

In the domain x R { m 4 } x \in\mathbb{R}-\left\{\dfrac{m}{4}\right\} , the range of the expression is R \mathbb{R} .


The answer is 1.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Let y = 2 x 2 5 x + 3 4 x m \large y = \dfrac{2x^2 - 5x +3}{ 4x -m} 2 x 2 ( 4 y + 5 ) x + 3 + m y = 0 2x^2 - (4y +5)x + 3 + my = 0 As x is real , d i s c r i m i n a n t 0 discriminant \geq 0 ( 4 y + 5 ) 2 8 ( 3 + m y ) 0 (4y + 5)^2 - 8(3+my) \geq 0 16 y 2 + ( 40 8 m ) y + 1 0 16y^2 + (40-8m)y +1 \geq 0 A quadratic in y is non-negative for all values of y if coefficient of y 2 y^2 is positive and d i s c r i m i n a t 0 discriminat \leq 0 ( 40 8 m ) 2 4 16 1 0 (40-8m)^2 - 4*16*1 \leq 0 ( 5 m ) 2 1 0 (5-m)^2 - 1 \leq 0 ( m 6 ) ( m 4 ) 0 (m-6)(m-4) \leq 0 m ϵ [ 4 , 6 ] m \epsilon [4 , 6] Consider m = 4 y = ( 2 x 3 ) ( x 1 ) 4 ( x 1 ) y = \dfrac{(2x-3)(x-1)}{4(x-1)} y = 2 x 3 4 , x 1 y = \dfrac{2x-3}{4} , x \neq 1 Since x 1 , y 1 4 x \neq 1 , y \neq \dfrac{-1}{4} Hence y cannot take all real values for m = 4. m = 4 . So, m = 4 m = 4 is not acceptable . Similarly m = 6 m = 6 is to be rejected . So for the given expression to take all real values , m ( 4 , 6 ) m \in (4, 6 ) . There is only 1 integer in this range.

Tricky problem and a very nice solution!

Ankush Tiwari - 4 years, 11 months ago

Log in to reply

thank you ankush and space

Ujjwal Mani Tripathi - 4 years, 11 months ago

Great solution .

space sizzlers - 4 years, 11 months ago

@Calvin Lin why has the problem been reported by multiple members ?

Ujjwal Mani Tripathi - 4 years, 11 months ago

Log in to reply

@Hung Woei Neoh , i have posted the solution

Ujjwal Mani Tripathi - 4 years, 11 months ago

@Hung Woei Neoh @Sal Gard @Rishabh Cool , i have posted the solution .

Ujjwal Mani Tripathi - 4 years, 11 months ago

Log in to reply

The issue with your solution is that quadratic discriminant has nothing to do with this.

Pi Han Goh - 4 years, 11 months ago

@Pi Han Goh , can you provide an alternate solution

Ujjwal Mani Tripathi - 4 years, 11 months ago

Log in to reply

Read the report section.

Pi Han Goh - 4 years, 11 months ago

The phrasing of the solution could be greatly cleaned up, similar to that of the problem. I've edited the problem, and it should now be easier to understand what is being asked for. Can you edit the solution accordingly?

Calvin Lin Staff - 4 years, 10 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...