( 1)∵.x= 1+2+3+4+5+6+7+88=4.5，。 y=6+6+7+3+5+3+3+78=5，8i= 1xiyi= 17 1，8i= 1x2i=204，

∴b=8i= 1xiyi？ 8.x.y8i= 1x2i？ 8.x2= 17 1？ 8×5×5204? 8×52=-7.25,

a=5-5×(-7.25)=4 1.25，

The linear regression equation between grade Y above ∴ 145 and class number X is y =-7.25x+41.25;

(2) The results of categories 7 and 8 are as follows:

? ? Excellent? Not good? Total? Class 7? 3 ? 42 ? 45? Class eight? 7 ? 38 ? 45? Total? 10 ? 80 ? 90∴K2=n(ad？ BC)2(a+b)(c+d)(a+c)(b+d)= 90×(3×38？ 42×7)245×45× 10×80= 1.8