# Linear Interpolation

I find the linear interpolation rule used in the schweser material different than the normal linear interpolation rule I use, can anyone explain for me the difference?

Liner Interpolation equation used in the CFA material=Interpolated rate = rate for lower bound + (# of years for interpolated rate – # of years for lower bound)(higher bound rate – lower bound rate)/(# of years for upper bound – # of years for lower bound)

the normal Lineral Interpolation equation=(# of years for upper bound-# of years for interpolated rate )*rate of lower bound+(# of years for interpolated rate-# of years for lower bound)* rate of upper bound/# of years for lower bound

A little algebra would show that they are equivalent expressions.

can you help me with deriving it?

https://en.wikipedia.org/wiki/Linear_interpolation

CFAI will come up with far worthier questions for you than deriving formulas for linear interpolation.

LowRate + (InterpolationPoint - LowPoint) / (HighPoint - LowPoint) * (HighRate-LowRate)

is the easiest way to think about Linear Interpolation.

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and that is the CFA formula. Going about it any other way is more complicated.