GIVEN:
x4+x41=2207
FORMULA USED:
(a + b)2 = a2 + b2 + 2ab
where a and b = 1st and 2nd term respectively.
(a + b)3 = a3 + b3 + 3ab(a + b)
CALCULATION:
Now, x4+x41 can be written as,
(x2+x21)2=x4+x41+2.x2.x21
Here, a = x and b = 1/x respectively.
⇒ (x2+x21)2 = 2209.
⇒ (x2+x21) = 47 ... (1)
Now, again (x2+x21) can be written as,
⇒ (x+x1)2=x2+x21+2.x.x1
⇒ (x+x1)2 = 49
⇒ (x +1/x) = 7 ... (2)
Now, take the cube of equation (2) on both sides, we get
(x+x1)3=x3+x31+3.x.x1(x+x1)
Here a = x and b = 1/x.
⇒ 343 = (x3+x31) + 3(7)
⇒ (x3+x31) = 322.
Now. (x7+x71) can be written as,
⇒ (x7+x71)=(x3+x31)(x4+x41)−(x+x1)
⇒ ( 2207 × 322 - 7) = 710647.
Hence, the value is 710647.
Shortcut Trick
We know, if x4+x41 = a, then x2+x21 = √(a +2),
If x2+x21 = b, then x+x1 = √(b +2) and if x+x1 = c then x3+x31 = c3 - 3c
Given, x4+x41 = 2207 then x2+x21 = √2209 = 47
So, x+x1 = √49 = 7 and x3+x31 = 343 - 21 = 322
Now, (x7+x71)=(x3+x31)(x4+x41)−(x+x1)
⇒ (x7+x71) = 322 × 2207 - 7 = 710647