Question

using identity is find the value (3a+9b) (3a-5b)

Answer 1

Hey ! ✌️

(3a+9b)(3a-5b)
=3a(3a-5b) + 9b(3a-5b)
=9a² - 15ab + 27ab - 45b²
=9a² -12ab - 45b²
=a² - 4ab - 15b²

Now solve the equation and you will get the answer.

Answer 2

Identity => (x + a) (x + b) = x² + (a + b)x + ab

Derivation:-

(x + a) (x + b)

=> x(x + b) + a(x + b)

=> x² + xb + ax + ab

=> x² + (a + b)x (by taking x as common) + ab




Now coming to your question :-

(3a + 9b) (3a - 5b)

By using the formula of (x + a) (x + b) = x² + (a + b)x + ab, we will solve the sum by putting the values.

Here, x = 3a, a = 9b and b = -5b


=> (3a)² + (9b - 5b)3a + (9b) (-5b)

=> 9a² + 4b × 3a + (-45b²)

=> 9a² + 12ba - 45b²



$\therefore{}$ (3a + 9b) (3a - 5b) = 9a² + 12ba - 45b²