设半径是r
扇形的圆心角是a
弧长=2*3.14r*a/360
周长2r+2*3.14r*a/360=28
r+3.14ra/360=14
面积=3.14*rr*a/360=49
a/360=49/3.14rr
r+3.14r*49/3.14rr=14
r+49/r=14
rr-14r+49=0
（r-7）^2=0
r=7cm
若扇形的周长为28cm,面积为49cm,求此扇形半径,
设半径是r
扇形的圆心角是a
弧长=2*3.14r*a/360
周长2r+2*3.14r*a/360=28
r+3.14ra/360=14
面积=3.14*rr*a/360=49
a/360=49/3.14rr
r+3.14r*49/3.14rr=14
r+49/r=14
rr-14r+49=0
（r-7）^2=0
r=7cm