(1)（sinx+siny）²=3/2
(cosx+cosy)²=9/4
两式相加可得
2+2（sinxsiny+cosxcosy）=15/4
2cos(x-y)=7/4
cos(x-y)=7/8
(2)设f(x)=ax²+bx+c
f(1)=a+b+c=2
f'(1)=2a+b=10
f(10)=100a+10b+c=11
a=-1 b=12 c=-9
f(x)=-x²+12x-9
f(5)=-25+60-9=26