(1)x≤150°,y=0.60x;151°≤x≤280°,y=150×0.60+(x-150)×0.65=0.65x-7.5;
x≥281°,y=150×0.60+130×0.65+(x-280)×0.90=0.9x-77.5;
∴y=
;
0.60x,x≤150° 0.65x?7.5,151°≤x≤280° 0.9x?77.5,x≥281°
(2)由(1)知,4月份,0.60x=73.8,∴x=123;
5月份,0.65x-7.5=155,∴x=250;
6月份,0.9x-77.5=264.5,∴x=380,
∴第二季度共用123+250+280=753°电.