Using this linear approximation we estimate that {{MATH_61}} is approximately {{MATH_62}} This estimate is an {{CMD_63}}{{CMD_64}}overestimate{{CMD_65}}{{CMD_66}}underestimate{{CMD_67}}{{CMD_68}} because {{MATH_69}} is {{CMD_70}}{{CMD_71}}concave up{{CMD_72}}{{CMD_73}}concave down{{CMD_74}}{{CMD_75}} between 5 and 7.