If `x=3\ a n d\ y=-1,` find the values of each of the using identity: `(x/7+y/3)((x^2)/(49)+(y^2)/9-(x y)/(21))`

Explanation

we know that,
`a^3+b^3=(a+b)(a^2-ab+b^2)`
using this formula,
we can write it as,
`(x/7+y/3)((x^2)/(49)+(y^2)/9-(x y)/(21))`
`=(x/7)^3+(y/3)^3`
now putting the values of xand y from the question,
we get,
`=(3/7)^3+(-1/3)^3`
`=27/343-1/27`
`=386/9261`