Error In Lagrange Interpolation Problem

Question

I have three points:

(1, 0.76), (1.3, 0.62), (2.2, 0.11)

And I need to find f(1.9) with Lagrange Interpolation so created this formula:

    P(x)=(x-1.3)(x-2.2)/(1-1.3)(1-2.2) 
+ (x-1)(x-2.2)/(1.3-1)(1.3-2.2) 
+ (x-1)(x-1.3)/(2.2-1)(2.2-1.3)

P(1.9) should be arround 0.29 but it generates very diffrent result. What is my mistake ?

Thanks.

Answer 1

You are not multiplying by the function value. For example, the first part of $P(x)$ should be $0.76 \cdot \dfrac{(x - 1.3)(x - 2.2)}{(1 - 1.3)(1 - 2.2)}$