Question

for what value of k will the equation X + 5 Y - 7 is equal to zero and 4 x + 20 Y + K is equal to zero represent concident lines

Answer 1

here is your ans

X+5y-7=0; ----------(1)×4;

4x +20y +k=0 ----------(2);

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4x+20y-28=0; -----------(1);
4x+20y+k=0;-----------(2)
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-K-28=0;
k=-28;
hope u like i

Answer 2

if the two equations have coincident lines then that shows that they have infinite solutions.
Now , in the case of infinite solutions - 
$a_1 / a_2 = b_1/b_2 = c_1/c_2$

on comparing and putting the values we get 

[tex] \frac{1}{4} = \frac{5}{20} = \frac{1}{4} = \frac{-7}{k} [/tex]

[tex]therefore \\ \frac{-7}{k} = \frac{1}{4}. \\ \\ -28 = k \\\\ [/tex]

Thus we can see
$k = -28$