linear regression [{1,0.25}, {2,0.75}, {3,1.25}, {4,1.75}, {5,2.25}, {6,2.75}, {7,3.25}, {8,3.75}, {9,4.25}, {10,4.75}, {11,5.25}, {12,5.75}, {13,6.25}, {14,6.75}, {15,7.25}]