1.若sin a+sin b=ㄏ2/2,则cos a+cos b的取值范围是多少?2.若cos^2A+cos^2B+cos^2C=1,则△ABC的形状为?“ㄏ”为 根号

【证明】Sin A+sin B=2Sin 22 证明sin(a+b)sin(a-b)=sin^2 a-sin^2 b, 求证:sin(a+b)sin(a-b)=sin^2a-sin^2b 已知sin(a)=2sin(b)求sin(a/2)/sin(b/2)的值. 在三角形ABC中,若(a^2+b^2)sin(A-B)=(a^2-b^2)sin(A+B),则三角形是?a/sinA=b/sinB=k则a=ksinA,b=ksinB代入(a^2+b^2)sin(A-B)=(a^2-b^2)sinC并把k约分(sin²A+sin²B)sin(A-B)=(sin²A-sin²B)sin(A+B) sin²A*[sin(A+B)-sin(A-B) 3 在三角形ABC中,已知（a2+b2）sin（A-B）=（a2-b2）sin(A+B) 求证：ABC是等腰或直角三角形(a^2+b^2)sin(A-B)=(a^2-b^2)sin(A+B),(sin^A+sin^B)sin(A-B)=(sin^A-sin^B)sin(A+B) sin^A*(sin(A+B)-sin(A-B))=sin^B*(sin(A-B)+sin(A+B)) sin^A*2c 懂得进.证明,sina+sin(a+b)+sin(a+2b)+...sin(a+nb)=sin(a+ab/2)sin[(n+1)b/2]/sin(b/2) 证明sina sin(a+2b) - sinb sin(b+2a) =sin(a+b)sin(a-b) 证明 sin^2A+sin^2B-sin^2A*sin^2B+cos^2A*cos^2证明 sin^2A+sin^2B-sin^2A*sin^2B+cos^2A*cos^2B=1 在△ABC中,求证；sin^(A/2)+sin^(B/2)+sin^(C/2)=1-2sin(A/2)sin(B/2)sin(C/2） 急 sin^A=sin^B=sinC(sinB+sinC)三角形ABC中,若sin^A=sin^B=sinC(sinB+sinC),则角A=呃……是sin^2A=sin^2B+sinC(sinB+sinC) 设A,B为锐角,且sin^2A+sin^B=sin(A+B),求证A+B＝90sin^2A+sin^2B=sin(A+B)sin^2A表示sinA的平方 已知3SIN^2A+2SIN^2B=5SINA,求SIN^2A+SIN^2B范围 △ABC中,若sin^2A-sin^2B+sin^2C=sinAsinC那么角B= 若arcsin(sinA + sin B) + arcsin(sinA - sinB) = π /2,求 (sin²A + sin²B) 的值. 若sin(A+B)sin(B-A)=m,则(cosA)^2-(cosB)^2=? 若sin^4a/sin^2b+cos^4a/cos^2b=1,证明sin^4b/sin^2a+cos^4b/cos^2a=1.(不用向量怎么证?) 在△ABC中,若sin^2A-sin^2B-sin^2C=sinBsinC,则∠A=