12. If 13^3+7^3/13^2+7^2-x= 20, find the value of x.b) 20c) 91d) None

1.	12. If 13^3+7^3/13^2+7^2-x= 20, find the value of x.b) 20c) 91d) None of thesea) 6
Answer» Answer: x = 91 Step-by-step explanation: GIVEN : To find the value of x, When, $\frac{13^3+7^3}{13^2+7^2-x}=20$ Solution : We KNOW that, $a^3+b^3=(a+b)(a^2-ab+b^2)$ By substituting, $a =13$ $b = 7$ & , We GET, $13^3+7^3 = (13+7)(13^2 + 7^2 - 13 \times 7) = 20 \times ( 169 + 49 - 91) = 20 \times 127$ By substituting in the given EQUATION, We get, $\frac{13^3+7^3}{13^2+7^2-x}=20$ $\frac{20 \times 127}{169+49-x}=20$ ⇒ $\frac{20 \times 127}{218-x}=20$ ⇒ By cross- multiplication, We get, $20 \times 127 =(218 - x) \times 20$ ⇒ $218 - x = 127$ ⇒ $x = 218 - 127 = 91$ ⇒