在三角形abc中,已知cosA=0.25,a+b=6,a+c=7,求c值
正弦定理：a / sinA = b / sinB = c / sinC = 2R ①
余弦定理：a² = b² + c² - 2bc cosA ②
方法2：② ===> a² = b² + c² - 1/2 bc
a+b=6 ===> b = c - 1
a+c=7 ===> a = 7 - c
(7 - c² = (c - 1)² + c² - 1/2 (c - 1) c
===> c1 = (-25 - √1009) / 2 (舍去負(fù)值)
c2 = (-25 + √1009) / 2
在三角形abc中,已知cosA=0.25,a+b=6,a+c=7,求c值