Question

扇形的周长为30cm,面积是56cm^2,则它的半径是________

Answer 1

C

设扇形的弧长为l

半径为r

则有

(r*l)/2=56 A

2r+l=30 B

由A得 l=112/r

代入B得

2r+112/r=30

2r²+112=30r

2r²-30r+112=0

r²-15r+56=0

1 -7

1 -8

-----

-7-8=-15

(x-7)(x-8)=0

x=7,x=8