LOS METODOS DE INTEGRATION 
43 
sen [ (a— a x )x + b — b l ] 
2 (a —a 1 ) 
sen [ (a + a 1 ) x + b 4-b' 1 ] 
2 (a-f-a 1 ) 
(Sturm, 359) 
25. f cos x cos 2 x cos 3xdx . Apliquemos la formula 
cos a cos b = cos (a + b) -r — cos (a - b) 
o 1 o 1 
. • . cos x cos 2 x= " cos 3 x + — cos x 
cos x+cos 2 x + cos 3 x== * cos 2 3 x + — cos x cos 3 x 
— Li 
pero — cos 2 3 x=l + cos 6 x 
0-1 , 1 2 
y cos x cos x= y cos 4 x -I — cos~x 
sen 6 x sen 4 x sen 2 x 
“6 + “"I + 2 +X 
(De Comberousse, 755) 
26. f sen x sen 2 x sen 3 x cl x 
sen a sen b = cos (a — b) ~~ cos (a + b) 
