15/16:24/10=15/16*10/24=25/64
X²(x²-1)-(x+1)=x²(x-1)(x+1)-(x+1)=(x+1)[x²(x-1)-1]=(x+1)(x³-x²-1)
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5ab/(a+5b)*(a²-25b²)/ab=5*(a-5b)(a+5b)/(a+5b)=5(a-5b)
a=5√7+9,b=√7-9
5*(5√7+9-5√7+45)=5*54=270
Sina=√19/10;0<a<90°
cos^2a=1-sin^2a=1-19/100=81/100
cosa=+√(81/100)=+9/10