` Delta ABC` में a= 7, b= 5 और c = 8 , तो cos 2B का मान ज्ञात कीजिए ।

Explanation

` cos B = (9a^(2) +c^(2) -b^(2))/(2ac) = (7^(2)+8^(2)-5^(2))/(2xx7xx8)`
` = ( 49 + 64 - 25)/112= 88/112=11/4`
अब ` cos 2B = 2 cos^(2) B - 1 `
` =2 xx 11/14 xx 11/14 -1`
` = 121/8 -1 = 23/98`