Question

x+y+z=15
xy+yz+zx=72
then the value of x is

Answer 1

you have given two equations only and there are three variables.  Hence one cannot solve for x as a numerical value.  But x can be found in terms of y and z.

     x + y + z = 15          --- 4

     (x + z) y + z x = 72
      (15 - y) y + z x = 72

          x = ( y^2 - 15 y + 72 ) / z  =  15 - y - z            ---  3

       then  y and z satisfy :  y^2 + z^2 - 15 (y+z) + y z +72 = 0      - -- 2

         you can solve for y in terms of z.

Further,
       (x+y+z)^2 - 2 (xy+yz+zx) = 15^2 - 2 * 72
                 x^2 + y^2 + z^2 = 81

                     81 - x^2  - 15 (y + z) + y z + 72 = 0

                   x^2 =  153 - 15 ( y + z ) + y z      --- 1

           substitue the value of y in terms of z,  then x can be obtained in terms of z.

Answer 2

Step-by-step explanation:

x+y+z=15

xy+yz+zx=72

then the value of x is