100-(307.3-20.4x):2.6=99.5;
260-307.3+20.4x=258.7;
20.4x=258.7-260+307.3;
20.4x=306;
x=306:20.4;
x=15
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1. 1-cos²(π/2-a)=1-cos(π/2-a)*cos(π/2-a)=1-sin²a=cos²a;
2. 1-sin²(3π/2+a)=1-sin(3π/2+a)*sin(3π/2+a)=1-cos²a=sin²a.
=(24/7)*(21/12)-1,7=6-1,7=4,3