` Delta ABC` में ` a= 9 , b=8 ` और c=4 , तो सिद्ध कीजिए कि -

Explanation

` cos B = (c^(2) + a^(2) -b^(2))/(2ca) = (4^(2)+9^(2)-8^(2))/(2xx 4xx 9)`
` (16 + 81 - 64)/72 = 33/72 `
` = cos C = (a^(2) +b^(2)-c^(2))/(2ac) = (9^(2)+8^(2)-4^(2))/(2 xx 9 xx8)`
` = ( 18 + 64 - 16 )/144= 129/144`
` cos B - 2 cos C = 33/72 - (2 xx 129)/144`
` = 33/72 - 129/72 = (-96)/72 = (-4)/3 `