`Delta ABC` में सिद्ध कीजिए कि -
` (b+c) cos. (B+C)/2 = a cos . (B-C)/2 `

Explanation

` L . H .S = ( b+ c) cos . (B+C)/2 `
` = ( k sin B + k sin C) * cos. (pi-A)/2`
` = k * ( sin B + sin C) * sin. A/2`
` = k * 2 sin. ( B + C)/2 * cos. (B-C)/2 * sin. A/2 `
` = k * 2 sin (pi-A)/2 * sin. A/2 cos. (B-C)/2 `
` = k * ( 2 cos . A/2 sin. A/2) cos. (B-C)/2 `
` = k * sin A cos. (B-C)/2 `
` = a cos. (B-C)/2 = R.H.S`