W r i t e space a n space e q u a t i o n space t h a t space e x p r e s s e s space t h e space r e l a t i o n s h i p. space space U s e space k space f o r space t h e space c o n s tan t space o f space p r o p o r t i o n a l i t y. r space v a r i e s space d i r e c t l y space a s space s space a n d space i n v e r s e l y space a s space t h e space s q u a r e space o f space t. r plus s minus t squared equals k r equals fraction numerator k s over denominator t squared end fraction r equals fraction numerator k t squared over denominator s end fraction r s t squared equals k