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甲、乙两小组同学用图甲所示的电路进行了一些实验探究.
(1)甲小组选取的定值电阻为15欧,电阻箱阻值如图乙所示,其值为________欧,此时电压表的示数为3.5V,则该小组所用电源的电压为________V;实验中若增大电阻箱阻值,电压表示数将________ (选填变化情况). (2)乙小组选用其它规格的电源和定值电阻进行实验,他们通过调节电阻箱,得到几组电阻箱的阻值和对应的电压值,并作出U-R图像(如图丙).由图像可知,电阻箱阻值为20欧时,电压表的示数为_____V,该组所选用定值电阻的阻值为____欧.
![](data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AAAA5ElEQVRYR2NkGGDAOMD2M+B0wKpVq9huv/jbxPD/fxwDA4MkhQ59zsDIuEhVgrkuLCzsF7JZOB3QOml5B8P//+UUWoyqnZGxszovsoI4B0xc9gyfzxmZmb2rcsK3IRvWMWm599///7fgcfTz6vwoKWId8B+f75kZGX0q8iK3kugAhur8KJRQxx0FE5fhdQC5UTPqgNEQGA2B0RAYDYHREBgNgaETAuTW96TqG7yt4tbRFhGRIQBqHTP9+8dIoDUMTxqk5AK8zXKYiaDWMYhNpANIaJYPdMdkwLtmpOZnctUPeDkAAP6tmiH1ggTLAAAAAElFTkSuQmCC)
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