)若f)在[a,b1上连续且为偶函数,则fx)dx=2fx)dx. (2)若)在[a,上连续且为奇函数,则二fx)dx=0. 证明:∫Cfx)dx=∫f(x)dx+∫fx)dx -(-x)dx+f)dx -[Lf(x)+f(-x)]dx. (1)fx)为偶函数时,fx)+f(-x)=2fx),故fx)dx=2fx)dx (2)fx)为奇函数时,fx)+f(-x)=0,故fx)dx=0. 例6若f(x)在[0,1]上连续,证明 (1)(sinx)dx-f(cosx)dx: om0d=引m,由d 证明:1)设x=7-4,则本=-d小,且当x=0时,1=号:当x=时1=0, sma=-/m传-]小-(cowr)d-(codx (2)设x=元-1,*/(sin.)xdx=(π-0/Isin(π-0d-0 (sint)d-(sin)dr 故(in)dx=fsin)d. 拥此公式可得:儿44 =-号1+dos 1 。g(co x啡号 xe,x≥0 例7设函数f(x)= i+osx-1sr<0te-2h. 解:设x-2=1,则(1)若 f (x) 在 [ , ] a b 上连续且为偶函数,则 ( )d a a f x x − = 0 2 ( )d a f x x . (2)若 f (x) 在 [ , ] a b 上连续且为奇函数,则 ( )d a a f x x − =0. 证明: ( )d a a f x x − = 0 ( )d a f x x − + 0 ( )d a f x x = 0 ( )d a f x x − + 0 ( )d a f x x = 0 [ ( ) ( )]d a f x f x x + − . (1) f (x) 为偶函数时, f (x) + f (−x) = 2 f (x) ,故 ( )d a a f x x − = 0 2 ( )d a f x x . (2) f (x) 为奇函数时, f (x) + f (−x) =0,故 ( )d a a f x x − =0. 例 6 若 f (x) 在[0,1]上连续,证明 (1) 2 0 f x x (sin )d = 2 0 f x x (cos )d ; (2) 0 xf x x (sin )d = 0 (sin )d 2 f x x ,由此计算 2 0 sin d 1 cos x x x x + . 证明:(1)设 x = − t,则dx = −dt 2 , 且当 x = 0 时, 2 t = ;当 0 2 x = 时t = , 故 2 0 f x x (sin )d = 0 2 sin d 2 f t t − − = ( ) 0 2 f t t cos d = ( ) 0 2 f t x cos d . (2)设 x = − t , 0 xf x x (sin )d = 0 ( ) [sin( )d( ) t f t t − − − = 0 f t t (sin )d − 0 tf t t (sin )d 故 0 f t x (sin )d = 0 (sin )d 2 f t t . 利用此公式 可得: 2 0 sin d 1 cos x x x x + = 2 0 sin d 2 1 cos x x x + = 2 0 1 d cos 2 1 cos x x − + = 0 (cos ) 2 − arctg x = 4 2 . 例 7 设函数 f (x) = − + − , 1 0 1 cos 1 , 0 2 x x xe x x ,计算 − 4 1 f (x 2)dx . 解:设 x − 2 = t,则