cos+x-y+等于
@钭姚955：COS(x+y)COS(x - y)=COS^2X - Sin^2Y
倪诗15294822513…… cos(x+y)=cosxcosy-sinxsiny cos(x-y)=cosxcosy+sinxsiny 所以cos(x+y)+cos(x-y)=2cosxcosy ---(1) 令x+y=2a ; x-y=2b 有: x=a+b ; y=a-b 故(1)式可写成 cos2a+cos2b=2cos(a+b)cos(a-b)---(2) 又 cos2a=2(cosa)^2-1 ; cos2b=1-2(sinb)^2 将此两式代入(2)式中即可得证
@钭姚955：证明COS(X+Y)COS(X - Y)=COS^2X - SIN^2Y -
倪诗15294822513…… COS(X+Y)COS(X-Y)=(COSX*COSY-SINX*SINY)(COSX*COSY+SINX*SINY)=(COSX*COSY)^2-(SINX*SINY)^2=COS^2 X(1-SIN^2 Y)-(1-COS^2 X)SIN^2 Y=COS^2 X -COS^2 X*SIN^2 Y-SIN^2 Y +COS^2 X SIN^2 Y=COS^2X-SIN^2Y
@钭姚955：若cos(x+y)cos(x - y)=1/3,则cox^2 - siny^2=? -
倪诗15294822513…… cos(x+y)cos(x-y)=1/2[cos(2x)+cos(2y)]=1/2[2(cosx)^2-1+1-2(siny)^2]=(cosx)^2-(siny)^2=1/3
@钭姚955：cos(X - Y)=0.5 sin2X+sin2Y=2/3 则sin(X+Y)=---------- -
倪诗15294822513…… sin2X+sin2Y=2/3,2sin(x+y)cos(x-y)=2/3,将cos(X-Y)=0.5代入,就可以了.用的是【和差化积】公式.注x,y与X,Y我没有区分开.
@钭姚955：1+cos^2(2x+3y - 1)=(x^2+y^2+2(x+1)(1 - y))/(x - y+1) 求实数xy最小值 -
倪诗15294822513…… 1+cos^2(2x+3y-1)=(x^2+y^2+2(x+1)(1-y))/(x-y+1)1+cos^2(2x+3y-1)=(x^2+y^2+2x+2-2xy-2y)/(x-y+1)1+cos^2(2x+3y-1)=((x-y)^2+2(x-y)+2)/(x-y+1)=((x-y+1)^2+1)/x-y+11+cos^2(2x+3y-1)=(x-y+1)+1)/(x-y+1)1|(x-y+1)+1)/(x-y+1)|>=2 所以只能取等于=2 这样...
@钭姚955：求证:cos(x+y)cos(x - y)=cosxcosx - sinysiny
倪诗15294822513…… cos(x+y)cos(x-y) =(cosxcosy-sinxsiny)(cosxcosy+sinxsiny) =(cosxcosy)^2-(sinxsiny)^2 =(cosx)^2[1-(siny)^2]-(sinxsiny)^2 =(cosx)^2-(cosxsiny)^2-(sinxsiny)^2 =(cosx)^2-(siny)^2[(cosx)^2+(sinx)^2] =(cosx)^2-(siny)^2
@钭姚955：求二元函数的偏倒数. 1.z=xy+ln(x^2+y^2) 2.z=cos^2(x+2y)(二阶) -
倪诗15294822513…… ^^1,z'(x)=y+2x/(x^2+y^2);z'(y)=x+2y/(x^2+y^2).2.z'(x)=2cos(x+2y)(-sin(x+2y)=-sin(2x+4y);z'(y)=4cos(x+2y)(-sin(x+2y)=-2sin(2x+4y); z''(xx)=-2cos(2x+4y);z''(xy)=z''(yx)=-4cos(2x+4y);z''(yy)=-8cos(2x+4y)..三.应用题1.解:联立x=y^2 y=x-2得交点为(1,-1)...
@钭姚955：求证COS(x+y)COS(x - y)=COS2x - SIN2y
倪诗15294822513…… zhengming: cos(x+y)=cosxcosy-sinxsiny cos(x-y)=cosxcosy+sinxsiny所以,二者相乘以后=cosxcoxcosycosy-sinxinxsinysiny=(1-sinxsinx)cosycosy-sinxsinxsinysiny=cos2x-sin2y
@钭姚955：matlab 有一函数 f(x,y)=x2+cos(xy)+2y ,写一程序,输入自变量的值,输出函数值. -
倪诗15294822513…… function f=fun4(x,y) f=x^2+cos(x*y)+2*y ;
@钭姚955：cos(x+y)cos(x - y)=2/5 求cosx方 - siny方 -
倪诗15294822513…… cos2x+cos2y=2cos(x+y)cos(x-y)=4/5 cos2x=2(cosx)^2-1 cos2y=1-2(siny)^2 所以cos2x+cos2y=2[(cosx)^2-(siny)^2]=4/5 所以(cosx)^2-(siny)^2=2/5
倪诗15294822513…… cos(x+y)=cosxcosy-sinxsiny cos(x-y)=cosxcosy+sinxsiny 所以cos(x+y)+cos(x-y)=2cosxcosy ---(1) 令x+y=2a ; x-y=2b 有: x=a+b ; y=a-b 故(1)式可写成 cos2a+cos2b=2cos(a+b)cos(a-b)---(2) 又 cos2a=2(cosa)^2-1 ; cos2b=1-2(sinb)^2 将此两式代入(2)式中即可得证
@钭姚955：证明COS(X+Y)COS(X - Y)=COS^2X - SIN^2Y -
倪诗15294822513…… COS(X+Y)COS(X-Y)=(COSX*COSY-SINX*SINY)(COSX*COSY+SINX*SINY)=(COSX*COSY)^2-(SINX*SINY)^2=COS^2 X(1-SIN^2 Y)-(1-COS^2 X)SIN^2 Y=COS^2 X -COS^2 X*SIN^2 Y-SIN^2 Y +COS^2 X SIN^2 Y=COS^2X-SIN^2Y
@钭姚955：若cos(x+y)cos(x - y)=1/3,则cox^2 - siny^2=? -
倪诗15294822513…… cos(x+y)cos(x-y)=1/2[cos(2x)+cos(2y)]=1/2[2(cosx)^2-1+1-2(siny)^2]=(cosx)^2-(siny)^2=1/3
@钭姚955：cos(X - Y)=0.5 sin2X+sin2Y=2/3 则sin(X+Y)=---------- -
倪诗15294822513…… sin2X+sin2Y=2/3,2sin(x+y)cos(x-y)=2/3,将cos(X-Y)=0.5代入,就可以了.用的是【和差化积】公式.注x,y与X,Y我没有区分开.
@钭姚955：1+cos^2(2x+3y - 1)=(x^2+y^2+2(x+1)(1 - y))/(x - y+1) 求实数xy最小值 -
倪诗15294822513…… 1+cos^2(2x+3y-1)=(x^2+y^2+2(x+1)(1-y))/(x-y+1)1+cos^2(2x+3y-1)=(x^2+y^2+2x+2-2xy-2y)/(x-y+1)1+cos^2(2x+3y-1)=((x-y)^2+2(x-y)+2)/(x-y+1)=((x-y+1)^2+1)/x-y+11+cos^2(2x+3y-1)=(x-y+1)+1)/(x-y+1)1|(x-y+1)+1)/(x-y+1)|>=2 所以只能取等于=2 这样...
@钭姚955：求证:cos(x+y)cos(x - y)=cosxcosx - sinysiny
倪诗15294822513…… cos(x+y)cos(x-y) =(cosxcosy-sinxsiny)(cosxcosy+sinxsiny) =(cosxcosy)^2-(sinxsiny)^2 =(cosx)^2[1-(siny)^2]-(sinxsiny)^2 =(cosx)^2-(cosxsiny)^2-(sinxsiny)^2 =(cosx)^2-(siny)^2[(cosx)^2+(sinx)^2] =(cosx)^2-(siny)^2
@钭姚955：求二元函数的偏倒数. 1.z=xy+ln(x^2+y^2) 2.z=cos^2(x+2y)(二阶) -
倪诗15294822513…… ^^1,z'(x)=y+2x/(x^2+y^2);z'(y)=x+2y/(x^2+y^2).2.z'(x)=2cos(x+2y)(-sin(x+2y)=-sin(2x+4y);z'(y)=4cos(x+2y)(-sin(x+2y)=-2sin(2x+4y); z''(xx)=-2cos(2x+4y);z''(xy)=z''(yx)=-4cos(2x+4y);z''(yy)=-8cos(2x+4y)..三.应用题1.解:联立x=y^2 y=x-2得交点为(1,-1)...
@钭姚955：求证COS(x+y)COS(x - y)=COS2x - SIN2y
倪诗15294822513…… zhengming: cos(x+y)=cosxcosy-sinxsiny cos(x-y)=cosxcosy+sinxsiny所以,二者相乘以后=cosxcoxcosycosy-sinxinxsinysiny=(1-sinxsinx)cosycosy-sinxsinxsinysiny=cos2x-sin2y
@钭姚955：matlab 有一函数 f(x,y)=x2+cos(xy)+2y ,写一程序,输入自变量的值,输出函数值. -
倪诗15294822513…… function f=fun4(x,y) f=x^2+cos(x*y)+2*y ;
@钭姚955：cos(x+y)cos(x - y)=2/5 求cosx方 - siny方 -
倪诗15294822513…… cos2x+cos2y=2cos(x+y)cos(x-y)=4/5 cos2x=2(cosx)^2-1 cos2y=1-2(siny)^2 所以cos2x+cos2y=2[(cosx)^2-(siny)^2]=4/5 所以(cosx)^2-(siny)^2=2/5
