If the first three Fibonacci nos. are x1=1, x2=1 and x3=2, what is the value of n for which xn + xn+1 = 89?

11

Step-by-step explanation:

Looking at Fibonacci numbers;

We have:

Than X1 = 1; X2 = 1; X3 = 2

Xn+1 = Xn + Xn-1

X3 = X2 + X1 = 1+1 = 2; X4 = X3 + X2 = 2 + 1 = 3; X5 = X4+X3 = 3+2 = 5; X6 = X5 + X4 = 5 + 3 = 8; X7 = X6 + X5 = 8 + 5 = 13; X8 = X7 + X6 = 13 + 8 = 21; X9 = X8 + X7 = 21 + 13 = 34; X10 = X9 + X8 = 34 + 21 = 55; X11 = X10 + X9 = 55 + 34 = 89

We see that; X11 gives 89 so the value of n = 11