Question

- 21 + 10r - r^2..factorise it ..pls answer​

Answer 1

Answer:

r = 3 and 7

Step-by-step explanation:

-21 + 10r - r^2 can be written as

-r^2 +10r -21 = 0

or

r^2 - 10r + 21 = 0 ( here i changed the sign of every term )

r^2 -7r -3r +21 = 0

take common..

r ( r - 7 ) -3 ( r - 7) = 0

(r -7) ( r - 3 ) = 0

hence ..

r - 7 =0

r = 7

or...

r -3 = 0

r = 3

therefore

r = 3 and 7

Answer 2

Given:

$\sf {r}^{2} - 10r - 21$

To Find:-

• value of r

Solution:

$\sf : ⟹  {r}^{2} - 10r - 21 = 0 \: \: \: \: \: \: \: \: \\ \\ \sf : ⟹ {r}^{2} - 7r - 3r - 21= 0 \: \: \\ \\ \sf : ⟹r(r - 7) - 3(r - 7) = 0 \\ \\ \sf : ⟹(r - 3)(r - 7) = 0 \: \: \: \: \: \: \: \: \\ \\\sf {\blue{ first \: zero : }} \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ \\ \sf : ⟹r - 3 = 0 \: \\ \sf : ⟹r = + 3 \: \: \: \: \: \\ \\ \sf {\blue{ \: second \: zero : }} \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \\ \\ \sf : ⟹ r - 7 = 0 \\ \sf : ⟹r = + 7 \: \: \: \: \: \:$

$\blue{ \underline{ \boxed{ \purple{ \sf{ \therefore \: r = 3 \: and \: 7}}}}}$

hope this helps.!!