sin 65derajat+sin 5derajat

sin^2 25' - sin^2 35' + sin^2 65' - sin^2 55'ingat bahwa cosx = sin(90' - x), maka= sin^2 (90' - 65') - sin^2 (90' - 55') + sin^2 65' - sin^2 55'= cos^2 65' - cos^2 55' + sin^2 65' - sin^2 55'= cos^2 65' + sin^2 65' - cos^2 55' - sin^2 55'Karena sin^2 x + cos^2 x = 1, maka= cos^2 65' + sin^2 65' - (sin^2 55' + cos^2 55')= 1 - 1 = 0